I want to book a United ticket to London using my Amex or Chase points. The outbound flight has PP which I want to buy. However, the return flight doesn't offer PP and I'm fine purchasing economy (and then upgrading). However, neither the Amex or Chase systems will offer me the first flight if I select economy class for travel nor will it offer me the second flight if I select premium economy. In other words, there seems no way to book a mixed class ticket on either system.

I can't book two one way tickets because United charges significantly more for one way tickets than round trip tickets on international routes.

Any other Ideas? I can book this on UA.com but then can't use my points.