The two holding plate configuration for the System 2M is possible but could present some challenges. Since you can only satisfy the needs of one holding plate at a time, it is essential that the pull down time of either plate not interfere with the cycle of the other plate. The key question will be how long is the cycle time for each holding plate and can those two cycles be interleaved with satisfactory results.
The concept is to build a daisy chain (or priority chain) using the contacts of digital thermostats along with solenoid valves to connect the plate being cooled to the compressor. The first thermostat in the chain is the highest priority and whenever it requests cooling it's solenoid valve would be open. Anytime the first thermostat in the chain is requesting cooling, the second thermostat is disabled and it's associated solenoid valve would be closed. Once the first thermostat has reached the setpoint the contacts of that thermostat would allow the second thermostat to request cooling and close the second solenoid valve.
This can also be done with mechanical thermostats but in that case additional relays are required to get enough contacts to control the daisy chain and the solenoid valves.
Which are you planning digital thermostats or mechanical thermostats? Please let me know and I will prepare a drawing and post it here.
RPARTS Tech Support