I assume the attraction of this arrangement was that it was quicker than designing a proper more conventional twin based on the Mustang, but the Wikipedia article claims there was a lot of new design, beyond, presumably, the new centre section. And by the way, it looks as though the guns would have needed to be synchronised in that position--is that so?
EDIT: I've found a three-view, which shows the guns located just outside the propellor disks, which would be the obvious way to do it: but it's a tight fit.
I'd be really interested in more information about the reasoning that led to this odd, but evidently successful, aircraft.