This has also been discussed in several threads over the years. I believe Timjet gathered some real world numbers and I've played around with it myself.
Yes I did post questions/comments on this several years ago. A member at the time came back with some very precise engineering data most of which was above my head.
To summarize in my case cruising at just below displacements speeds and for me that was 1300 rpm and 8 kts when I shut one down my speed dropped by only 1 kt to 7kts. So I thought that it was definitenly a fuel savings. In fact I thought that if you note your speed and rpm with both running and shut one down as long as your resultant speed is greater than half the speed with both running then there is a fuel savings running on one. Not so.
As I understand and perhaps some experts will chime in, a diesel engine will try and maintain a set rpm at a set throttle position. So when one is shut down, the other engine will increase fuel flow to maintain it's rpm, in my case 1300 rpm. How much more fuel is used can only be really determined with fuel flow meters.
Also hull design, prop pitch, rudder size and the amount of rudder deflection required to counter act the assymetrical prop thrust all come into play and are all different for each boat.
However on the other side of the equation any engine will require a certain amount of power that does not go to the prop, coolant pump, raw water pump, transmission, turbo, that all take power away from doing any work, - power and fuel that is required just to operate the engine but not making the boat go.
So bottom line you won't know for sure unless you have accurate fuel flow.
Do a search with author timjet around the 2011 time frame and you will see some excellant info on this subject.
I decided to operate on 2 engines, it's safer especially when running in narrow channels and fuel is cheap.
Don't worry about ruining a diesel engine by not running it hard enough. That doesn't happen to much, more likely is early failures due to over propping. With those 500 hp engines you mentioned you can get up on plane when needed, but save fuel and cruise at displacement speeds the rest of the time, that's what I do.