Nearly a year after “Grey’s Anatomy” made television history by becoming the longest-running primetime medical drama, the ABC series is taking on a new challenge on Thursday, Jan. 23: becoming more immersed with its spinoff, “Station 19,” for a more unified two-hour weekly block.

But while there are natural ties between the two dramas, trying to line up storylines, share casts — who film on different lots — and find the right balance for the shows is a constant juggle.

“It was for sure the greatest challenge of my career and everyone around me has been saying the same thing,” Krista Vernoff, who runs both shows, tells Variety. “[‘Station 19’ director/executive producer] Paris Barclay’s walking around saying, ‘I don’t know why we don’t have documentary crews rolling. This is crazy what we’re doing.’”

Here, Vernoff breaks down how the worlds of “Station 19” and “Grey’s Anatomy” will collide.

Syncing the Shows

Much like the first “Grey’s Anatomy” spinoff, “Private Practice,” which ran from 2007-2013, “Station 19” was launched as backdoor pilot on the mothership in 2018. Though there was an obvious overlap — “Grey’s Anatomy’s” Ben (Jason George) became a firefighter and moved to “Station 19” but remained married to “Grey’s Anatomy’s” Bailey (Chandra Wilson) — the two shows remained fairly separate.

But with the request for “Grey’s Anatomy’s” 16th season and “Station 19’s” third year to be more interconnected, Vernoff started from the ground up.

“My top priority was character work and finding stories that excited me,” she says. “So I had to start at the beginning, which is falling in love with the world of first responders, the characters, figuring out what I still wanted to know about them and what I wanted to discover.”

And once those things made themselves known, she realized that the relationships, both platonic and romantic, would help serve to unite the shows. “That’s where you start in Shondaland,” she says.

Complicating matters was “an avalanche of insanity”: although the shows needed to be on the same timeline, “Station 19” was held until midseason. That meant “Grey’s Antomy” was tasked with setting up new relationships, like the unexpected romance between “Station 19’s” Vic (Barrett Doss) and “Grey’s Anatomy’s” Jackson (Jesse Williams), while not completely spoiling “Station 19’s” cliffhangers.

Production-wise, “Station 19” was a few weeks behind the mothership in episode number, but the “Grey’s Anatomy” hours were airing much sooner — which meant the actors on “Station 19” were filming far ahead on relationships that had just begun to develop on “Grey’s.”

“[Doss and Williams] were playing scenes on ‘Station 19’ without knowing what happened really between them on ‘Grey’s Anatomy’ the eight episodes prior,” Vernoff recalls. “They’re calling me and going, ‘We’re having this fight, but what has happened? How close are we? How long have we been together?’ And the two writers’ rooms, side by side, have some answers to those questions.”

The unusual schedule meant that the writers arced out “Grey’s Anatomy” significantly deeper into the season than they normally would, Vernoff says. And it’s made negotiating crossover guest stars — and their schedules — more complex than years’ past.

“When we book a guest star for ‘Station 19,’ we have to pin those guest stars and make sure they’ll still stay available for the episode of ‘Grey’s Anatomy’ that won’t shoot for two months,” Vernoff explains. “And they can’t change their hair [in between]!”

Though a number of other series share combined universes — including the CW’s “Arrowverse” and NBC’s trio of Chicago shows from Dick Wolf — the sheer scope of connectivity has made this process an unparalleled challenge. For guidance, Vernoff has leaned on her former “Shameless” colleague John Wells and “Grey’s Anatomy” creator Shonda Rhimes.

“John Wells, who has sort of seen and done everything, just has a way of helping me remember that it’s not brain surgery,” Vernoff says. “It just quiets me down a little internally. Shonda just laughs and thinks this is delightful to watch me go through [this].”

Vernoff was a long-time producer on “Grey’s Anatomy” before she took the reins as showrunner, but her tenure on “Station 19” is newer. The show itself is still in its early stage, and this is her first time running it.

“There’s not a model for it because of the particular set of circumstances,” she says. “It’s a new showrunner [for “Station 19”] in Season 3 and because of the amount of merging of the two worlds that we’ve been asked to do. What I do is marvel at the way the cast, crews, producers and writers have risen to this extraordinary task. The fact is that we’ve sort of been asked to accomplish the impossible and we feel like we’re pulling it off.”

Combining the Worlds

When it came time to craft the major crossover that kicks off with “Station 19’s” season premiere, Vernoff was faced with creating a story that would simultaneously resolve “Grey’s Anatomy’s” fall finale cliffhanger and “Station 19’s” second season finale cliffhanger, introduce potential new viewers to the world of “Station 19,” and move the story ahead to match the timeline of the mothership.

“The challenge was to honor and respect the fans who’ve been watching for the first two seasons [of ‘Station 19’],” Vernoff says. “Re-piloting would be easy. If I’d been able to bring ‘Station 19’ back simultaneous to ‘Grey’s Anatomy,’ I wouldn’t also be faced with bridging the nine-week timeline. So it was a challenge; it was daunting.”

But, Vernoff says she is “excited by where it landed.”

“I feel like we I feel like we touched on almost every cliffhanger that the show left in in season two,” she continues. “We went deeper into some of those cliffhangers; we handled some of the others in one line of dialogue. And we created an exciting hour of TV, where you meet these firefighters doing what they do. What they do is thrilling and completely different than what surgeons do. I wanted to invite you into a world to have you meet these people fresh, if you were fresh, and be excited to see them again if you weren’t.”

Ending “Grey’s Anatomy’s” 2019 run with a car crashing through Joe’s (Steven W. Bailey) bar also served as a natural bridge into “Station 19.”

The executives at ABC “were so excited when we pitched the car through the wall,” Vernoff says. “They understood instantly what that did: It very organically asked people to come watch the other show to see our [‘Grey’s’] people get rescued. And they were so excited by that concept.”

The scribe also praised executives for not artificially forcing storylines or actors to appear on either show that don’t genuinely fit in with the plot the writers are trying to tell. “The network has been extremely supportive and extremely respectful,” she says. “They understood the size of this challenge. There has not been a lot of pressure to force dynamics.”

With network production schedules a whirlwind under the best of circumstances, the shows weren’t able to add days to their calendar to allow for more wiggle room for series regulars to hop between shows. (Though, with a number of series regulars from each show recurring on their sister series, the producers were granted “a little bit of extra financial support” to help accommodate that burden.) Vernoff credits the “Grey’s Anatomy” and “Station 19” line producers with keeping the show’s schedules running smooth amidst the madness.

New Players, New Rules

Although the Shondaland universe has been very good about going back to actors they liked — Joshua Malina, Katie Lowes, George Newbern and Bellamy Young appeared on both “Grey’s Anatomy” and “Private Practice,” while Scott Foley and Jeff Perry had big arcs on “Grey’s,” prior to starring on “Scandal” — the new combined universe means fresh ground rules for guest stars.

“I discussed with Linda Lowy and her extraordinary casting teams that if they’ve appeared on ‘Grey’s Anatomy’ in the last two seasons, if they’re [going to be] on ‘Station 19,’ they have to be playing the same character,” Vernoff says. “I checked that with Shonda because I was like, ‘What are we doing here [with rules]?’ And Shonda basically said, ‘You see the same actors over and over on ‘Law & Order’ playing different characters, it is what it is. We’ve been on the air for 16 years, we’ve used every actor in Hollywood, let the actors work.’”

However, as interconnected as the shows are, they still have to be created for a viewer of either drama to be able to have a complete story if they choose to just watch a single show per week.

“The best experience is going to be to watch both,” Vernoff notes. “You’re going feel like you got more stories if you’re watching both — like you have a more complete world. But here’s another angle to this challenge: These shows only air together on ABC on Thursday nights. When you stream them, you’re not watching them this way. Overseas ‘Grey’s Anatomy’ is in a lot of markets that ‘Station 19’ isn’t [in yet].”

This means that “each hour of each show needs to be a whole story [by] itself,” she continues. “And when merged with the other show, when there are crossover elements, it needs to feel like, ‘Oh, that’s a bigger movie.’ So I had to make two-hour movies, [but] if you only watched [either the first or last] hour, you felt totally satisfied.”

Ongoing Effects

Crossovers will continue throughout the season, though on a much smaller scale.

Shifting “Grey’s Anatomy” from its Thursdays at 8 p.m. time slot to 9 p.m. also allowed for “Station 19” (which now airs at 8 p.m.) to introduce characters who will go on to be patients “either the same night or sometimes the following week,” Vernoff says.

“We’ve got an episode [of ‘Station 19’] where we’ve got a camping trip and there’s an incident,” Vernoff previews. “And the whole hour of TV is this really fun, funny, delightful episode of ‘Station 19’ that culminates with a patient who we care about and will follow through on [over at] ‘Grey’s Anatomy.’”