As the maintenance person and skipper of a multihull with the kind of drive system you are describing, I guess I have a couple of observations I can share:
1) There are no cost or weight savings - the whole rig tends to weigh more than two small engines. A true dual system is inherently more redundant than a single engine driving two props.
2)The hydraulic system is mybe 80% efficient at best (transmission losses due to the pumps, motors). If you go with expensive technology like adjustable pumps, etc. then you can have better control, torque even at low prop speeds. We have variable displacement pumps on our cat, work well enough as long as you don''t mind the $1500+ maintenance costs every 5 years or so when they get serviced.
3) If the bypass pressures are not set correctly, a snag in the prop will kill the engine. Our bypass valves were set to 4000 psi - 1500 psi above what the engine could develop without stalling. Damage to the pumps also results (see maintenance bill above).
4) Stock up on filters and oil. Your hydraulic system will invariably have parts not accessible anywhere you are going. Our metric stuff takes 3 weeks in the US from time of order to arrive. God help you if the pumps or the motors fail. The average turnaround time increases to two months in that case. Carrying spares isn''t really an option (unless you''re rich and don''t mind the wait).
5) Depending on how the hydraulics are mounted, they can cause more noise than the engine. The brain-dead designers at Diport AG in Switzerland specified that we use saildrives (mistake #1) and mounted them directly to the skin of the hull (mistake #2). Saildrives are fragile and ours now hangs lower than the hull, ensuring that whatever we hit, we hit first with the saildrive. As for mounting the saildrive to the hull, imagine the noise that a high-speed pitchy whine produces throughout the structure - it''s louder than the engine!
6) Have a look at the propulsion part of my web-site. I go through all the troubles we''ve had with our system in gory detail <a href=http://www.vonwentzel.net/Prout/01.Propulsion/index.html>at http://www.vonwentzel.net/Prout/01.Propulsion/index.html</a>.
7) Having said that, some people do benefit from such drives, monohull racers in particular. They never use their engine unless they have to, and the hydraulics allow them to mount the engine where it''s closest to the desired center of gravity.
I''d stay away from such systems. No matter how interesting and "good" they look on paper, they required skilled mechanics, not found everywhere. Small saildrive aux engines are a better bet IMHO.