Open Jaw means you won't be able to MFU on-line. What you can do is check availability and then 'phone BAEC? Its a pain but its a well-known draw-back to Open Jaws, especially in WT+ and the darned phone fee then wipes out much of the value (typically for me as I book for two or three of us on the same booking).
However, you should be able to select and pay for your seat each way via MMB. If not then it may be the system has not fully processed your (over the phone?) booking yet; sometimes it can be >24hr wait.
If you pay for a seat and then do get an MFU, you forfeit the seat fee paid (unless you chase your losses by paying for a CW seat!!)..