I'm thinking Russel did that to address to issues, the curator in the 50th special, and the Renegade Doctor. I know a lot of fans, m'self included, want her to return, but how do you explain her? Well, if the doctor bifurcated... Sorry, bi-generated in the past, willingly or unwillingly, that could mostly explain her, and if 14 and 15 are running around at the same time, no reason she can't either. Thinking about the curator, that might even explain why Russell brought Tennent back.