Good luck. I tried booking the following flight in one reservation, and couldn't, unless I split the trip into two separate reservations.

IAD-FRA in paid J, FRA-SPU in Y, SPU-FRA in Y, FRA-IAD in paid J.

No matter what I tried, I couldn't do it unless I booked IAD-FRA r/t in paid J on one reservation, and FRA-SPU in paid Y on a second reservation. When I called UA to ask what I was doing wrong, I was told that on the new site, each-way pricing doesn't easily allow for the scenario I wanted to purchase.

I was told that from time to a time a "mixed cabin" option might pop up that fits my needs but that will be the exception not the rule. I was told that my best option was segmenting the trip into multiple reservations.