You may wish to measure your fuel consumption at different speeds, just to make a data sheet for yourself.
From the photo in your first post, I doubt that you have enough keel to move your boat at less than a crabwise angle while running on a single engine, so you will have a lot of rudder required to go where you want to go. That will increase your fuel consumption, likely well past any savings from the reduction of friction losses by running only one of your two engines.
Weebles recollections bring back my own recollection of his example. In the same era, or close, we had a GB 42 owner on this forum or its Passagemaker predecessor, who ran on one and provided a data set, comparing single efficiency to twin, with his own results showing more fuel consumption on the single to achieve the same speed.
My own data set, accumulated when I was forced to spend a season on one, showed me that, for my own boat only; the single could not achieve the usual cruising speed comfortably; The speed achieved when running at the usual rpm was just below hull speed, where the fuel consumed by the single was unchanged from its consumption while the twin was running at that same rpm, just the speed through the water decreased.
The most indicative bit of data was the height of the stern wave. In order to make a wave of the height I make cruising at 8 knots, the single couldn't do it comfortably. Making the wave at a slower speed could be done on significantly less fuel, but there is no huge difference whether running one or two.
Pushing the boat from the aft corner required a lot of countering rudder, so more fuel consumption than keeping the rudders strait, this in a full keel design. Yours without a full keel would require more rudder still.