I have to always call and work with someone in reservations; my airline tickets have so many damn combos to make it work, I can never pull it off via the website alone.
They have been super helpful though when booking like this. On our last trip, it was actually a bit cheaper for my son's tickets to be two one-way options vs. a standard round trip. So, it was miles for one ticket, companion fare for the second, then two one-ways for the third....and we made it to and from Baja with no issues (well, the only big one being I didn't catch a Roosterfish...)