There might be an issue with the remote turn-on lead. Some radios don’t provide enough current out of the remote lead to turn on multiple amps. Try running 12V from the battery to both of the amps remote turn-on leads and see if they both turn on. If that works, then you can use a relay connected to the radio’s remote turn-on lead and switched 12V power to switch your amps on. Also check to see if you get a voltage drop when both amps turn on at once. It’s possible that one of the amps is going into protection mode due to under-voltage. There could be a wiring, car battery, or alternator problem that isn’t supplying enough current for all your car’s electronics.