I was just watching an episode from 1974, Series 4, of the best TV show of all time (The Brothers), on DVD, and it featured 2 actors who would, in due course, play the role of Doctor Who : one, of course, was Colin Baker playing regular character Paul Merroney ; the other, playing an Investment Fund Manager, was none other than Richard Hurndall ; I recognised him but couldn't place him until the end credits rolled.
I wonder what other examples there are of 2 (or more) future Doctor Whos turning up in the same production?
There's quite a few. Hartnell and Troughton were both in a 1948 movie called "Escape", Hartnell appeared with Troughton in a 1953 film called "Will Any Gentleman...?", and there was an episode of "All Creatures Great and Small" where Troughton and Davison appeared together (although that one is a bit of a cheat as Troughton had long since stopped being Who...)
If you're happy to count Richard E Grant (Scream of the Shalka/Curse of Fatal Death), there's Withnail and I
David Tennant was in the Christopher Eccleston-starring film Jude. (I think they just about appear in one scene together.)
Peter Capaldi and Jodie Whittaker were in the anthology series Accused, which also starred Eccleston (and Capaldi having already been in The Fires of Pompeii), but I'm assuming they were in separate episodes.
William Hartnell and Jon Pertwee (not Patrick Troughton, as posted by Simon Collis earlier) were in the film Will Any Gentleman...?
Am now on the last series (7) of The Brothers, on DVD, and have just watched an episode where Colin Baker's character, Paul Merroney, refers to women in Istanbul as 'looking like Daleks'. Well, it amused me, anyway!
One future Doctor and one past Doctor here, which, although outside the original parameters, I thought might still be of interest. Jodie Whittaker and Christopher Eccleston talk about the National Theatre's 2012 production of Antigone in which they both starred (with Whittaker in the title role) - it also includes footage of them together in the production: youtu.be/A9-W66xB-fM