https://www.digitalspy.com/tv/a37205393/doctor-who-peter-capaldi-return-special/

"I think the more of the multi-Doctor stories you have the less effective they are, really."

it's been traditional for them to have multi-Doctor stories for the Christmas specials, and that's pretty much the only time they have them other than the regeneration episodes.

honestly I think that any potential Doctor should be bound to a minimum 3-5 seasons and required to come back for at least 1 out of every 4 multi-Doctor specials - otherwise David Tennant will be in every single one of them. I'm tired of getting a new Doctor every two seasons and want them to stick around longer.