If it is a UA codeshare on LH metal, you will not be able to upgrade it in advance. However I found out that you can upgrade at the gate if spcae is available (just did it yesterday at FRA with eVouchers). You need 2 eVouchers or 50,000 miles per person and segment. In my opinion it is a waste of miles, since you get a round trip award ticket in BIZfor 90,00 miles
If it is UA metal, you will not be able to upgrade
Cheers, Stefan