This has not been consistent. In "Time Crash" written by Steven Moffat the Fifth and Tenth Doctors meet each other. The Tenth Doctor is able to remember the events of "Time Crash" as he has already lived through them.
However, in "The Day of the Doctor" written by Steven Moffat the War Doctor, Tenth Doctor and Eleventh Doctor all meet. At the end it is stated that the War Doctor and the Tenth Doctor will both forget the events as soon as they leave, as when two or more incarnations of a Time Lord meet up, only the most recent incarnation(in this case the Eleventh Doctor) will remember the events.