Therefore, its known narrative elements are not a part of the Doctor Who universe as we, on this Wiki, choose to define it. It may have been the basis for a similar story in another medium, however — and that story may indeed be valid.
Fifty-Fifty or 50/50 was an unproduced audio story planned by Steven Hall and featuring a "big, epic and pretty brutal showdown" between the Seventh and Eighth Doctors, with Ace, Hex, and Lucie being forced to choose sides.
50/50 would have relied on a very different ending to the Black and White trilogy: the revelation that the Old Gods and other monsters were returning because something travelling very fast was smashing holes through the dimensions. This object would have been revealed to be the bruised and battered Eighth Doctor, who would tell Hex that the Seventh Doctor had caused it and that they were fast approaching "the moment where 7 went bad."
As Hall was planning for this story when he wrote A Death in the Family, many character moments in that story were designed to build toward the showdown between the two Doctors. However, because of the characters involved and the actors the story therefore needed, it was deemed too expensive and never produced. Hall suggested it might fit in with a "new generation" of The Lost Stories.