Ding there are gear boxes sold by some hobby stores that are made for what you want to do.
The closest thing I could find in the 5 minutes i lookes was a 2 motor to 1 prop gear train sold by hobbypeople.net. I bet you could reverse this and get a 1 motor to 2 prop thing going.
To be honest there is no advantage in haveing a 1 motor to 2 prop plane. Basicaly by going to 2 props your gona be drawing way more amps, that'll kill your run time. The only way around this is to get bigger batteries (much heavier) or go to smaller less pitch props. Then your back where you started, but have a hearvier drive train. If your looking for more speed try uping the cells and going to a higher pitch prop. if its more low end power you want up the cells and go to a larger size but lower pitch prop. If you need two props for some other reason the i would suggest going with 2 motor systems. you can still go off the same battery. If you run the motors in parrallel it will still be a good trade off in wieght vs power. if you go in series you'll burn em up before you get off the ground.
