On twitter, people started debating between 10 and 14 but I just see them as a set. Tennant's casting made it easier to see them as a pair.
14 works as a book end for 10 rather than a seperate incarnation. Ironically you could've used 14 to bring in the Valeyard by having two tennants run around. One is 14 while the other becomes the Valeyard.