I think Mikey would be left handed, but I don't have proof that he is. It's just a feeling, and most left handed people are very creative and silly like Mikey is XD.
I think Leo could be left handed too. Because when you hold one sword with two hands, doesn't the dominant hand go on top of the other hand? And I think I've seen Leo's left hand on the top. But I'm not really sure.
Donnie could be both, or he could have taught himself to use both so he could work better or something like that.
And, I'm not really sure about Raph. Maybe he's both, so he can use both his weapon's, but still I'm not quite sure.