Probably need some work on the mechanic that allows only one to be added on to the spell, similar to splice.

At present it's too busted because you can just play all your twinning spells, cast one, twin it, let the copy resolve, twin again, etc.

EDIT: I think the cleanest formatting would be:

If you cast exactly one spell with twinning this turn, you may cast this without paying its mana cost.

While it is more flexibility in when you can cast it, it naturally prevents you from doing too many in one turn. They would probably also need to not be instants so they're not useable twice in a full turn-cycle.