There is "Doctor Who Unleashed" episodes for each of the specials that is the behind the scenes and gives a bunch of the backstory to them getting back together. I found them on torrent sites so not sure if they are available from standard sources since it was air on BBC. The way Russel wrote the third episode we have 2 Doctors now and there could be an entire new series of the Dr/Donna and the continued adventures. The split regeneration was something he always wanted to do.