Ok, so you've got 2 completely independent circuit's, 2 x condensers, 2 evaps or 1 x split evap ?
Is the oil in the faulty system definitely the correct type ?
Is the oil heater working ?
Does the oil loss occur in both heating & cooling ?
If you're going to run the compressor that looses oil, make sure the oil level is above the min level before starting. Do not do this too many times as any excess oil will have to be drained.
Also do not either start or allow a compressor with excess oil in to run..
As i said, if you can't get the 'missing' oil to return, don't keep trying to run the compressor.
You're then going to have to drain it from wherever it is in the system, even if this means a decant..
:)