Some guys have done it out here. Duck hunters aka duck boat racers. A pair of 15's is going to be heavier (significantly) than a single 30, thus you gain nothing and lose quite a bit.
On larger offshore stuff, they can get away with multiple outboards because there's no such thing as a reasonably priced, reasonably lightweight 600hp, 800hp, etc outboard. In fact, back when Yamaha introduced the F350, it was exactly what that motor was geared towards, offshore rigs that would benefit more from a single 350hp as opposed to twin 175's, or twin 350's vs 3 250's, etc. I think now Evinrude has a 400, same principle.
I personally don't see any need for it, and in fact unless they're tied together, for me it'd be tough to control both motors synchronously and safely. I certainly wouldn't not try it, but I'd be really reluctant to do it on waters I couldn't stand up in...