Two into one has been around for years. Back in WW2, some landing craft
used 4, 6-71's connected to a single shaft through a clutch arrangement.
I'd bet the problem will be to find someone doing this on a regular
basis for a boat the size you want ... i.e., anything can be engineered,
but the cost and benefits may not compare.
Turbo electric, diesel electric, steam electric is also common on larger
applications ..... BG Mayhaps some of the "hybrid" technology for cars
will soon find it's way to more boats.