They're not *supposed* to coexist with themselves (or as I prefer to say it, loopback their personal timelines), according to the episode with Rose's father, but apparently that's just to reduce the risk of paradoxes, not an automatic temporal problem. There were certainly a couple of old episodes where different Doctors worked together, weren't there? And Jack was talking like he comes to the Blitz and Pompeii over and over again, which is also brain-hurty, but, still.