You should be able to call after you have made the reservation and sort this out.

Note, I haven't done this exact thing, but I am MP Gold which allows me and a companion E+ at booking. Not a problem if paying for both mine and my wife's ticket, however last year I paid for my ticket and used miles for my wife's ticket (hence separate reservations) and obviously couldn't choose an E+ seat for my wife since she has no status. Quick call to the premier line, gave both PNR's and they moved my wife to the empty E+ seat next to mine.