Filling the reciever will make for interesting start up. Remove reciever and install cap tube drier or some other access for highside. In the 80's I serviced an ice bank bulk tank with captubes and recievers. The recievers were due to length of evap. tubes. Thus large volume of refrigerant required to flood
One of the machines is a Virtis Ultra LE 25, has daul compressors with receivers and caps.
Cooling multiple loops (shelf/condenser/neg. shelf) at different times during the run, sometimes more than one loop. Varying loads - a small receiver helps reduce the time spent pushing vapor through the cap tubes... This is what I've always assumed.
Of course, anyone doing Ultra Low temp long enough knows that some of the normal rules get ignored.
The shelf compressor only cools the shelf, it has two caps. One cap is for cooling, one is for when the recipe calls for no cooling. The condenser compressor cools the condenser of course, unless the switch is off. If the shelf is on and the condenser switch is off the condenser compressor still runs and supports the shelf cooling.