I have flown LAX->FRA->JNB a couple of times using United and Lufthansa. I believe it is about 100,000 miles to upgrade to business class because you do need to use two Award tickets. In fact the process was a little complicated when I did it, but perhaps the situation has changed now.

I had to call Lufthansa and check on availability of seats, then request upgrade coupons and finally try to use those for the upgrades I wanted. Some of the upgrades did not clear until the day of the flight so it can be disconcerting to be unsure about your flight status until you arrive at the airport.

If you end up using United across the Atlantic, you will likely have a long layover in Frankfurt. I used the time to explore the city and found it relatively easy since there is convenient train service directly to and from the airport.

John