Well, that had the advantage of never having been tried. I feel like they could've been a bit more overt in the scene on the helipad that the Fifteenth Doctor is the Doctor and not, like, the Doctor's son. The scenes in the TARDIS were a bit more clear, but even so, those almost feel like a leftover from an earlier draft, where Fifteen just popped back in time normally, and there was a normal regeneration from Fourteen to him that happened off-screen sometime in the years to come. That line about Fifteen being fine because Fourteen took a break is what I'm thinking of, as presented, it doesn't make sense, they're the same age, they only split as people a few hours earlier and had been together in the meantime (Donna also makes a comment about Fifteen being older, rather than being Fourteen's twin).
(Personally, I think the Toymaker actually killing the Doctor mid-episode would've probably been much more effective dramatically than either of those possibilities, but it looks like my dream of a totally surprising, shock regeneration remain unfulfilled.)
I think the first half of the episode was stronger than the resolution. A couple laugh-out-loud points to call out, a blink-and-you-miss-it shout of an old lady looting a giant TV during the chaos montage at the very beginning, and Kate when she was under the influence of the Giggle accusing Bingham of malingering since she'd moved her legs, just like the internet CHUDs a couple weeks ago. I suppose that reaction was predictable enough that RTD could make a called shot a year in advance depicting them as being insanely delusional because of too much screentime, but it was still pretty impressive. Speaking of, though, the TV-makes-us-worse theming (it feels wrong to call it "subtext") was pretty on the nose, and while RTD's take on Who always had a streak of cynicism, I started to think the Doctor was under the influence of the Giggle himself when he began ranting in UNIT HQ about how the Giggle was merely exposing the pervasive depravity of the human race. A bit harsher than the old "dumb apes" and "pudding brains."
RTD does add some fuel to the idea of the Timeless Child being a trick of some kind, with the Toymaker taking credit for turning the Doctor's life story into a jigsaw puzzle, and implying that he was using or impersonating the Master. I wonder who, exactly, is in that tooth. There were theories that the Spy Master might've actually been younger than Missy, or maybe even Saxon, since he didn't seem to be aware of any of her character development, but, now, it's possible the Spy Master never actually existed at all. And, of course, we've got another red-nailed hand saving the Master. It'd be a nice change if that ended up being someone guessable, rather than a red herring like last time where it was obviously Lucy Saxon, and ended up being a new character we'd never heard of before. Anyway, as far as the TC goes, much like Blade Runner, I find it far more interesting if the Doctor fears they might not be who they think they are than that they actually aren't who they think they are.
Also, all the standard potential story hooks from having a spare Doctor still apply. One could become the Curator while the other never retires, one could become the Valeyard (they're all technically between the Doctor's twelfth and final regeneration, now!), or we could just have David Tennent playing three distinct Doctors in the 70th anniversary special, just like the joke poster.