If you are booking a companion ticket, if it works like the UK Amex 2-4-1 you don't seem to be able to get all 3 PAX on one PNR when the 2-4-1 voucher is used (you have to book those two seperately from the standard miles redemption ticket) - this may affect the seating arrangements.
Unless anyone knows different (I did manage to do this online several years ago, but have failed in the last 2 years both online and over the phone).