Why it is necessary to have both a test separator and a group separator in an oil battery? I realize that you need to meter the fluids, but can't you just do that after the group separator and save a vessel and some piping?
Production must be allocated to individual oil-wells. You establish your allocation criteria by sending the individual wells to the test separator. Then you sum the oil, gas, and water production from the tests and divide each well's well test production by the total of the Well Tests to get the portion of the total production that can be attributed to each well.
If there weren't regulatory, contractual, and engineering reasons for the test separator it wouldn't be there.