The whole 2 v 1 argument is VERY subjective, I've had RIBs with inboard, single outboard & twin outboard (current) all had advantages & dis-advantages. For sheer economy & out & out grunt, the Ribcraft 585 with the 175 Ficht was more fun than a barrel of frogs
. However, when we started venturing further afield (20+ miles offshore) my missus insisted that we buy a new boat with 2 engines, who was I to argue
The current boat with twin 115s on a 6.4m Ribcraft is very capable, it feels safe & stable on the water & is very surefooted in the rough. The trade off is higher fuel consumption & slightly lower top end speed when compared to the 5.85/175 combo. Personally if we were just pottering about & not diving offshore I would probably go for the single option. But for the added security I do like the twins. I have twin tanks that can be connected via a valve, twin Optima batteries with changeover switch. I have the option to run both engines from 1 tank or 1 engine from both tanks. Similarly I can start / charge either engine from either battery.
The choice is yours & depends on your approach / attitude to security v cost. There's no point going down the twin route unless you do it properly & that means adding around 50% to the cost of a single engine/hull setup.
Oh! & BTW they DO sound most excellent when you get the revs matched spot on