Since each side of the motor is a seperate cooling system, it's a good idea to monitor both sides in any way possible, either by two seperate gauges or with the original two sender system. It sounds like your 2-pole sender was malfunctioning. As was mentioned earlier, the 2-pole is just a temperture on-off switch with a bi-metalic temperature sensitive trigger. If you can find a good used one or try one of the repops, it could get your system working again.
You can test them with several jumper leads and a sauce-pan with water on a hot plate. That and a decent thermometer to check the water temp against the gauge. Just make sure the senders are grounded to a common ground with the indicator and the battery source.
Kerby