I don't think that, for now, we can make a connection based on it's desgin.
It's true that they bear similarities, but while the Majora's Mask housed a very powerful entity, and it's used turned you into a puppet, the Fused Shadow only gave you tons and tons of power, even though, that power is not comparable to one third of the triforce, since Ganon destroyed it with his bare hands.
I think that, until a game proves that there is a connection, any coincidence must be ignored.
Also, don't forget that, as far as we know, Majora's mask is orginary from Termina, while the Fused Shadow is either from Hyrule or the Twilight Realm.
And, on a sidenote, I am pretty sure that we'll hear of the Fused Shadow in the Future, don't forget that in the ending of TP, it's remains layed half-buried on the Hyrule Field, and there was an oddly suspiciously close-up to it.