so, you are going to replace 2 12 hp engines with one 15 hp engine driving a
generator (losing efficiency), which in turn drives two electric motors (losing
efficiency).

Now how in hell is that an improvement?

The 47 foot, 20,000 lb. cat my numbers are from used two 12 horse engines.
Running off a 10 kW
genset, it could make 7 knots. So your numbers are, typically, off by a
factor of three.