All AS fares are one-way fares. UA does look to have some roundtrip fares on route (second letter in fare basis code is an 'F' instead of an 'A'), but they aren't really much cheaper and you can't combine them with AS fares. The cheapest UA fares on route that allow combinations with other carrier fares look to be Q fares (QAO7JKEN and QAA7JKEN). These fares have a 7 day advance purchase requirement (4th position in fare basis code). The difference in third position of fare basis code is due to different day of week travel requirements. So if there is inventory in lower UA fare classes and you meet fare requirements for them, it can actually be cheaper to purchase AS and UA one-way fares separately.