If you have two engines you could be running at 40% each, or shut one down and run the other at 80%, it will be more efficient to do the latter (unless you have

If you have two engines you could be running at 40% each, or shut one down and run the other at 80%, it will be more efficient to do the latter (unless you have to be able to start the idle one quickly and that's not possible if it's at 0%, so then you run one at 75% and the other at 5%).