so i showed M, and we had a small discussion on the topic! here is what she had to say:

"so due to a lack of ability to really differentiate ghost and vivi's combat from each other (no small thanks to them having the same goal of Please Dont Kill Me) and of course the fact that we are likely not going to be able to implement a more sophisticated switching system and worries of depicting switches in a less than flattering manner they will not have gameplay differences
there will be different voice clips for each one, and some small animation changes that will not affect gameplay"

basically: certain moves with voice clips in ghost's voice or vivi's voice, winposes and such for each of them, maybe a cofronting one, etc.
i initially wanted to make vivi a kind of echo fighter as well but we decided that it would be too difficult to sprite and code such a thing ;;
a lot of extra work for functionally the same characters gameplay-wise, when it would be more in character for them to work in sychronicity instead!