While Phantom slash is love and life in Smash 4, It would NOT be worth the trouble to separate the characters in P:M.
First, we have to make them new moves. P:M is already in some warn water with Nintendo, so basically designing something akin to the moves in Smash 4 would be a no no, simply to avoid any issues. But this would suck, and I would have LOVED to see what they could do with Bouncing Fish, Burst Grenade, and Phantom Slash, but they really can't use those moves.
So then what do they do for those moves?
Then we have to think of the spectrum of
imbalance that would have to be thoroughly addresses when ONE WHOLE MOVE gets added to two very heavily and fully developed characters. There would likely need to be some sort of tone-down to the character that no one will like if the move is to have any utility at all, because why even create an entire new move just for it to be useless? The P:M definitely wouldn't want to do THAT. (Note: One of these characters is SHEIK. NOTHING will truly ever be done to alter her, so nerfs would be out the window, BUT THEN so would any good sides to a new down B, because she's already good enough a new move would end up being to much unless it was basically a taunt lmao.)
Just my 2 cents looking at it. There would be a lot of mess to possibly add moves that really can't be there without bringing on mounds of complaints. If anyone wants they can feel free to post this elsewhere, but I don't think I'll be taking part in actual discussion, partially due to how rusty my P:M knowledge is, and
I've already had my share of reading tiresome Zelda arguments.