Good luck, I've had experience doing what you are attempting to do, and it's simply no fun with UA.

Your big friend in all this (if you want to attempt it yourself instead of a Travel Agent) is ITA.

If you can create the itinerary in ITA, then UA should be able to piece it together.

I tried booking a trip from EWR-BHX-FRA-JNB-PLZ in W (to use two SWUs [one on LH]) and because the final SA leg was not in W, but in S, the UA website would only piece the whole itin together in S, making the SWU invalid on both legs. So, I booked placed the itin on hold, called up and asked the agent to work on it. About 2 hours after starting, while being on hold the entire time, they came back and basically shook their shoulders and said - 'well, i'm not sure how to do it, I guess it can't be done'. And that was it.

So, I tried myself for a few more hours and was able to finally get it to work by trying multiple iterations of the destinations to combine some segments while forcing others...but of course, I did it myself and without any assistance from UA. If I remember correctly, I finally did it on UA's (this is after the merger, so CO's website): EWR-BHX-FRA-PLZ, which allowed me to isolate the UA segment on EWR-BHX, the BD segment on BHX-FRA and keep the fare class consistent on the LH segment (V) for FRA-JNB but also lumped in the JNB-PLZ segment in S. I also chose the specific fare classes of S, W, V in the option...and luckily it worked for me, but UA wasn't any help...and that frankly was not a surprise.

-jeremy