I wish I'd thought of it - my wife and I have the same first and last names - but the FF statements have gender specific salutations ("Mr." and "Mrs."). The trick would be to both use "Dr." or "Rev." or "Rabbi". Except one then would wonder what to do in an on-board medical or religious emergency - would I have to reveal the truth or claim a Ph.D. or lapsed pulpit? Actually the only concern one should have is what the airline can do if they catch you. Theoretically they could revoke all miles and ban you from their flights. Would they?