Most cruising cats seem to have two engines, one in each ama. This is absurd as it is twice as complex as it shoud be. So, why not a single engine driving a hydraulic pump. Then there could either be a simple hydraulic motor in each ama or a single hydraulic motor in the middle. You would have simpe hydraulic lines running from the pump to each motor. The motors could easily be driven separately.
For that matter, a single centered hydraulic motor could be on a system that can be raised from the water for less drag.
Single engine twin screw hydraulics:
I toyed with hydraulics in my 51' cat in the 1980's, and determined it was not worthwhile. I used little Vickers landing gear pumps mounted to twin retractable legs that held I/O bottom end/prop, and used standard twin Volvo MD2's with stock hydraulic pumps mounted. This removed the props from the water tucking them into the bridgedeck. Problem with hydraulics is that they are extremely noisy..... noisier than the diesel engine itself. Next is the heat, and will require an additional heat exchanger. Then there is hose chafe, and was always breaking a hose every 6 months, despite my best attempts to chafe proof it and use of ss braided protection. Then there is the loss of horsepower... like 20% at EACH pump. One motor to one prop still = a 40% hp loss. Now if you want to use one motor to power two props, there are tecnical problems, like prop RPM control..... you cannot use standard twin screw manuvers, because both the forward and reverse props will be spinning at the same speed, whick can cause handling complications. There is a way to spin your props and different RPMS in foreward and reverse, but this would require the use of a very expensive and very heavy rotary valve, with all of it's attenuant controls, heat, and yet another 20% power loss. Also, single engine hydraulics to twin props will never give you the same redundance safety as two separate engnes and props, which is far more reliable, especially with their own transmissions and direct fdrives. Hydraulics in the end will not save any weight either, when taking all into consideration. Note: the retractable drives were not a good idea either..... they slowed the boat down radically when down, eliminating the possiblity of effective motorsailing.
If you are hell bent of trying someting new, worth pursuing may well be diesel-electric, using DC generators insteasd of propulsion engines, in which one could be shut down when powering at slower speeds, and this would also cover all power requirements. The mounting of the genrators would be not as critical as propulsion engines, and could be set on larger rubber dampeners.