heres my idea.. use 1 single gauge, an adjustable temp warning light setup and 2 real senders, a special relay ( can't think of the type/ name ) that will switch off 1 power source when another is switched on, and otherwise work just like a normal relay.. this way, if your 'second side' starts getting hot enough to turn your adjustable warning light setup on, that circuit will close the relay and it will switch senders, so ither way, it can be seen if it is getting hot