Piggybacking air tanks

I think you may have a problem... Presuming both compressors start at the same time when there is no pressure in the system,
they will run until one of the pressure switches shuts its machine down. The second machine could then maintain enough pressure to prevent the first machine from restarting. If you mess with either of the pressure switches and force a compressor to start at full system pressure, you stand a chance of burning out the motor.