You can use one of the temp gauges to watch two readings if you go with Autometer short swept temp gauges that have a single wire going to the sending unit. I have Autometer sport comp, short swept gauges that only have one wire going to the sending unit.
You will need a on/off/on switch, and another sending unit. The autometer short swept gauges take either a short sending unit probe (#2258) and/or a long sending unit probe (#2259). With the on/off/on switch, the center terminal will connect to the gauge. Then on the end terminals, one will go to a sending unit and the other terminal to the other sending unit. You will be able to flip the switch from one sending unit to the other.