Going slow youd get about 5mpg on diesel, say about 5-6 knots,it would be good as well if you could trim the legs independently to reduce drag, ie run one engine at a time.
(these figures are for diesels)
And what happens if you try to run a twin screw boat on only one engine? - it wants to go in circles and you have to fight the wheel continuously to try and maintain a somewhat straight coure - this is not an option in passagemaking and would only be appropriate if you have lost an engine due to mechanical problems and need to limp to port.
I used to run my 1960 32' Chris Craft Connie w/ Corvette Ns on one engine at 5-6 kn all day with just a little rudder on her and sippin' gas(the boat, not me). At full throttles, her nine tons would come up on plane and she'd do about 30, with the fuel gauge needles sinking like a lead weight.