If people can't use SDL1 for the moment with pygame, then we will be in a situation where two branches are needed. Rather than that, it seems simpler for me with a single code base for SDL1 and SDL2.
I guess this is the core of it. To me, the single code base sounds harder than keeping two separate branches, especially as I expect the SDL 1 branch wouldn't see many changes. But you're the one doing it, so if you prefer the single code base option, that's reason enough to do it that way. ;-)