The original solenoid setup is working fine but I’ll be replacing it all with a new parts. The Viair 485C Revision 2 dual compressor kit came with a solenoid (active relay).

I will still use a manual toggle switch under the dash to activate the solenoid which in turn will provide +24vdc to both compressors. One of the tanks will be fitted with a pressure switch that will send a signal to the solenoid to disengage the compressors voltages when the maximum amount of air pressure has been achieved.