Steven Bogos said:
Doctor Who fans might remember that the last time we saw the tenth Doctor, he created a half-human half-time lord clone of himself and sealed him away in a parallel dimension with companion Rose Tyler. It's highly likely that the 50th anniversary storyline will take place in the parallel universe, as it's the only logical explanation that would see both Smith and Tennant appearing alongside each other (as well as the appearance of Rose).
Using logic when talking about Doctor Who? That's crazy!
William Hartnell, Patrick Troughton and Jon Pertwee met each other in "The Three doctors" (1973)
In "The Five Doctors" (1983) Richard Hurndell (Playing the first doctor as William Hartnell had passed away), Patrick Troughton and Jon Pertwee and Peter Davison all met. Clips of Tom Baker were in the Five Doctors but he did not meet any of the others.
In "The Two Doctors" (1986) Patrick Troughton and Colin Baker met each other.
In "Time Crash" (2007) Peter Davison met David Tennant.
All of the above didn't have half-human half-Time Lord clones to meet. The TARDIS is a finiky thing when it comes to time lines. Also the Doctor's enemies and the Time Lords themselves have all been responsible for bringing various incarnations of the Doctor together in one place.