If the XLSForm / XForm combination doesn't seem like a good idea, we don't have to do it. I'm personally getting confused moving back and forth between different feature types and spec types so I believe it would help me think more clearly. I can imagine others may approach them in a different way or perhaps may only care about one spec but not the other.
Another option is that we could say all spec change proposals get made in parallel on the two different repos and perhaps appearance changes always go to the pyxform repo only. This is roughly what has been happening recently although often the change on one side gets accepted before the other conversation completes.