Well i can only think of 2 situations that their could be 2 final smashes, One if their are 2 smash balls and Samus gets the first and then her other form gets the other and if Zelda and Shiek are together cuasing them both to have different ones
i'd say that two smash balls appearing at the same time isnt common. and if there were two final smashes for each character, how could you trigger one or the other? it's not like you can press B to trigger both FSs... which button would trigger the other smash?
With the info we have so far we know that each character has ONE final smash. Im pretty i also heard that if you get a smash ball as ZS Samus you turn back into normal Samus.