Wait, is ultimate Roxas the one who wields 2 keyblades?

And for thread's sake, here is a picture of Anti-form Sora, for whoever may not have come across him yet.

No, Ultimate Roxas is the one that can use the keyblades in a special way; he can make them float in air and attack in whatever position he wants.

And he has some badass looking moves.

The boss one, that's just made so he has the Twilight power on all the time and he's more aggressive against Sora.

In the final mix+, right? Roxas in 358 days has a wicked limit, especially when he uses 2 keyblades.

plus, when fighting DW roxas, you can steal his keyblades and command them to your will (sora's will) for brief periods of time. it's a key part to beating him in FM

But that won't help much once he begins the Meteolaser attack.

And Ultimate Roxas, if I remember correct, keeps spamming Meteolaser which makes it hard to reach him at all.

What is the story behind Anti-Form? Why does Sora transform into this form? (I don't want the technical meaning, like you have to bla bla this many times, I'm talking about story-wise.)

Sora transforms into Anti-Form because he was once a heartless. The darkness didn't completely leave and now it reappeared to turn Sora into Anti-Form. Also because of the darkness that Roxas is, Sora can turn into Final Form.

So they basically are related to heartless and nobodies, but have nothing to do with them.