Seeing as a couple of people have mentioned that Catherine Tate was
at the table read the other day, and Piper's been officially confirmed, it seems pretty obvious that 11 runs into 10 (both of him) during the events of "Journey's End", somewhere after 10 has dropped off Jack, Martha, and Mickey, but before he drops of Rose, Jackie, and the human version of himself, and before
he mind-wipes Donna.
So 11 will get two interact with at least 3 other Doctors (10a, 10b, & "Doctor Donna").
Plus, I'm still
betting on appearances by David Bradley as Doctor 1, and Reece Shearsmith as Doctor 2.