[Sans Peur]

You dont say what engines you have but, get an IR gun and see what the temps are comparatively on each engine. It should be easy to find any hot spots. Especially look around the heat exchangers. They may have not been maintained and need cleaning.
Another easy check is water flow. Could need an impeller on one side (but do both, they most likely have the same amount of hours). Start them.up.and see if the hot engine flows about the same as the other side.