yes/no situations happened for me on LH with LH's miles&more and UA's MileagePlus in 99 (for upgrades from business to first, I don't know for free tickets which I never use):
on LH transatlantic flights availability of upgraded first class seats using UA miles was ok (for Gisela and me flying FRA-LAX for the PIP) , when there was no more availability using LH miles. The flight-segment itself was paid in business (as part of a StarAlliance rtw-ticket) as a LH-flight-number (and LH was the actual carrier too).