Can American and United flights be combined on one ticket?

Whether American and United can be ticketed together depends on the rules of the specific fares in question. Many fares, particularly the heavily discounted fares, cannot be combined for these two airlines. It is important to note that this is different from the American/Delta situation. When American and Delta terminated their “interline” agreement in 2015, not only did they cease to allow their flights to be ticketed together, but also stopped the automatic transfer of luggage and do not use each others’ flights to protect displaced passengers. American and United have maintained their interline agreement and continue to rely on each other in these ways. For either set of airlines, if your schedule is best accommodated by using one airline in one direction and the other airline for the return this can be handled by booking two one-way tickets.