How much fuel depends on speed, of course. At WOT, probably pretty close to twice, I'd guess. At cruise, well, I suppose it depends on how you're measuring, but for a given speed, I'd guess about 1.5 times (that's a guess, though.)
You would have much more available power, but you're also adding a lot of weight, and increasing lower unit drag by a factor of 2, so the tradeoff may not be as good as you'd think.
Most of the references I've seen say you lose about 1/4 to 1/3 the total rating going with twins (i.e. your twin 50's would net you the equivalent of a 66 to 75 hp single.)
This is all theoretical, as I have never played with twins (I do like the way you can crab the boat around with twins, however. That would make it worth it in my book. Then again, I'm not paying for it, either.