Mega Man's Mega Buster

[tsukiyomaru0]

It's whichever hand he wants it to be.

BINGO! Because both hands supports the Buster.
Now, a neverending question: Why can't you use your other hand's buster as you have fired his arm with MM7's Rush Adapter or MMV's fully charged shot?

[Disco]

Not enough RAM

[Mr. X]

For Mega Man, that's the equivalent of patting your head and rubbing your stomach at the same time.  He just can't do two things at once unlike some other robot masters.