Depends. In the UK, the dramas Casualty and Holby City are set in the same hospital. So it isn’t unusual for staff from upstairs (Holby City) to be seen down in the Emergency Department (Casualty) or the other way around. Or even the ED being seen on Holby City. And that has been happening from 1998, since some Holby City characters and characters debuted on Casualty, prior to the launch of Holby City in 1999. Despite Casualty being filmed in Bristol and then Cardiff, while Holby City is filmed at Elstree in Hertfordshire. Most crossovers have been cases of characters from one show appearing on the other. Other cases have been major crossovers, starting on one show and concluding on the other. But they also did a storyline where a character from Holby City was romantically involved with a Casualty character and that was about fourteen crossover episodes between the two shows, culminating in the near death of one of the characters and the explosion of one of the Casualty regulars. As well as some storyline sharing and multiple instances of a character from one show transferring to the other. Henrik Hanssen from Holby City has guested in fourteen episodes of Casualty. And characters have been regulars on both…Mike Barratt, Nick Jordan, Connie Beauchamp, Lofty and Fletch. The character crossovers increased in the last decade.
Although it was fun on Holby City when they were still accepting patients from the ED, when it had been blown up on Casualty and I guess they didn’t notice.
And there was a third series, Holby Blue. Charlie Fairhead has been in the first episodes of all three shows. Jac Naylor has also appeared on all three shows.