Which is most Economical/Beneficial to (a) to always run both compressors in tandem or (b) use the second compressor when the outside temperature drops to O°c (for example) to assist the 1st compressor?

If it was my system I would run it with a two stage thermostat. Set it up for time and temp diff. to bring on second stage. If the system runs longer than 15 min second stage will kick in or if you have more than a 2 degree swing in temp second stage will kick in. It cuts down run time of the system in turn will save you money.