I have one temperature gauge on my truck on a three-way switch that I use to monitor engine oil, transmission, and differential temps. It works just fine - whenever I want to check on the temps I just slide the switch through the three positions. However, my gauges are aftermarket (Autometer), and are electrical gauges (not mechanical) that use a sensor that varies the resistance on the ground wire that goes to the gauge. If your transmission temp gauge is a factory gauge, knowing what vehicle you have would help folks make appropriate suggestions.
Good luck!
Regan
