I'm still trying to decipher what has been said. One temp sender for a different aftermarket temp gauge and both temp gauges attached to that one sender? I don't think that will work. I dont know what the aftermarket gauge has for power supply. If on a 12 volt supply, what back feed issues might occur?
If a good fuel gauge gets the right voltage in and through it, 23/24 ohms should take that needle to 1/2. Move that blue wire to in place of the purple temp sender wire / pin and that temp gauge should go to half also. That could prove your aftermarket temp sender and gauge are the root of problems. good luck with it.