Try grounding the wire going from dash to first sender if it moves the wiring to dash is ok.
Next try grounding the second sender if it moves that sender is the issue if not the other sender is.
Elimiate the 2 wire sender, hook to the single wire sender it is the one that does the work. I bet it is the one wire sender, if it is remanufactered sender they go bad pretty often, try to get an old orginal one they seem to last forever
Your original temperature gauge is a King Seeley thermal type to be used with a King Seeley points regulated sender. The gauge should start on HOT and travel to COLD when the key is energized. As the coolant temperature sender heats up, the internal points make contact less frequently thus causing the gauge pointer to return towards HOT. Essentially they work in the opposite fashion of "modern" gauges. Did you replace your senders with original units? Tested the temp switch sender?