take the sending unit wire from one engine and run it over to the other motor, see if the temp difference moves with it or stays with the motor. if it moves, you have a problem with the gauge or electrical connections to the gauge, if not either the sender is bad, the electric connection to the sender or the engine temps really are different. if the temps are actually off by 10 degrees there is a problem in the cooling system somewhere that needs attention. its either one or the other and each can be frustrating to find, but it is a good idea to run it down and fix it before you have bigger trouble.