Very doable. As you board just swap boarding passes. It might be courteous to explain to the FSM what you've done (look for the member of staff in the black/grey shirt) and if it's a female FSM then you'll likely get major brownie points and who knows a bar invite might be forthcoming later in the flight. Not guaranteed though . Deciding which leg wil be the tough call . Day flights are more fun as overnight people tend to sleep.