Considering that Moffat is full of shit 68% of the time, I'm having trouble believing this actually happened, and if it did, how committed David and Matt were to actually DOING it.
It's one thing to talk about it between takes and have them go
"This will be fun!"
"We should totally do it!"
...and another thing to actually do it. Now, as much as I love Tennant -- I started with Eccleston but consider Tennant "my" Doctor -- he had his time on the show and left. If he had come back to do this, it would've felt like it was infringing on Matt's time. And how would two leads affect the budget over a series? And would they have brought back any of 10's companions who would agree to this?
I don't think it got very far. I think Moffat is exaggerating.