It’s a great question. I haven’t looked at this much. Most of my extras ran OK in 3.0 when I tested them, though that was at least a year ago.
My inclination would be to use a single version that either works on both versions of MODX or checks the version number and behaves accordingly whenever possible.
Whenever I’ve created multiple versions of something, I’ve been sorry. There are always changes that need to be applied to both packages. There’s something really annoying (to me at least) about having to make the same changes twice (changing the code, changing the version number, updating the changelog, uploading, describing and releasing the new version, etc.). YMMV.