you really dont want to have an idle compressor in the same refrigerant circuit while one runs... check valves end up in refrigerant and / or oil getting trapped causing either a false 'overcharge' or false 'undercharge' condition depending on where the refrigerant and oil end up trapped.. you also run a high risk of compressor slugging..
im not sure how many BTU of cooling you are trying to achieve.. if you fitted 2 single ton inverter minisplits (there are ones made to run on 12 volts for solar applications).. you could then install a 200 - 250 amp alternator on your engine and drive one or two of those units.. you could easily fashion an RPM switch so that if the engine RPM's drop below a certain point only one of those units will run.. (unless your alternator has a high output at idle or you have a good size battery bank to use as ballast)..
Mini split tyle units are soft-start so there's no compressor surge...
NOW the CAVEATS!.
minisplits have an outdoor unit thats a bit sizeable..
muinisplits use R410A which runs at significantly higher pressure..
Minisplits are not generally for mobile use..
HOWEVER you can see where im going with this.. running electric compressors all the time and using the engine to generate the electricity as opposed to 2 separate compressors..
12 volt A/C compressors... - NOT CHEAP but the thermoking tri-pac systems use an electric compressor.. I believe you can get up to 12,000 - 15,000 BTU from one..
Transair used to make one.. (not sure if they still do)
Climax just released an Inverter type small all-in-one soft-start air conditioner that albeit runs on 110 volts is quite efficient and has a High COP for heating and high EER for cooling...
and finally some people go the route of installing standard RV A/C on the roof and then running their generator when driving... thats the SIMPLEST way to get A/C while on shore and highway power..