I'd prefer it if they did something like that to bring back Paul McGann. I want more Eighth Doctor in live action, darn it!
When I think about the Tennant/Smith idea, I'm torn. At first glance, it sounds really freakin' awesome! But I feel like doing it for an entire 13 episode season might be straining the novelty of it.
I've been thinking lately that, if they ever do a year like 2009 where they just do a bunch of specials in lieu of a full season, they could have each special feature a different Doctor. Like, one each for McGann, Tennant, Smith, & Capaldi.
BTW, much as I love Matt Smith, I'm really looking forward to Capaldi. I've been watching & reading a lot of 3rd Doctor stuff lately, so I'm even more predisposed towards an older Doctor anyway. He's even been popping up in some of my dreams lately. Fall 2014 can't come soon enough!