I answered no opinion/don’t care but I don’t think that’s entirely accurate, just closest. I don’t mind it as a one-off thing. As they said it’s a Time Lord legend, something not thought to have actually happened before, a legend that was able to happen because of the Toymaker’s influence. As a one-off, I think it’s an interesting idea, and something to make that particular regeneration unique. Regeneration has never really been consistent or the same over the course of Doctor Who, and bigeneration is just one of many differences in the Doctor’s regenerations, a difference I think a lot of fans have overreacted to.
That said, I don’t want it to become a regular thing, frankly don’t want to see it ever happen again, and certainly don’t want it as a permanent change to regeneration. RTD has said speculation/expressed his belief that now all Doctors have bigenerated, which I think should be ignored, and absolutely should not happen/be revealed to be true. I think the idea that all things must end and change to move on is a very important theme of Doctor Who, and if regeneration just became bigeneration, I think that theme would be lost. The other thing is that I subscribe to the time loop theory, where 14 lives out the rest of his life, and then at the end becomes 15 as he’s sent back into the bigeneration “the long way round.” I don’t want the bigeneration to lead to a whole different second Doctor.