...but why couldn't 9 show up and hang out with 10? Why couldn't we have a mini-season called Doctors Who and there could be squeeing and awesomeness for six glorious episodes? And bring Cap'n Jack, too. Just for funsies. Man. 9, 10, and Jack. That would be the BEST party.
There's probably some dumb paradox law that Timelords have to obey, but if I were 9 or 10, I'd say, fie on thee, rules, I'm the LAST effing Timelord, and I want to hang with myself, because myself is awesome.