I was able to book and ticket the following award:
LAX-SFO (UA)
connecting to
SFO-FRA-LHR (LH)
stopover
LHR-CPH (RG)
six months later:
CPH-FRA-LAX (LH)
This an F award. UA agent told me that with the *A awards, I could have either one stopover or one open jaw but not both. In my trip, I have the LHR stopover.