The best advice I can give you is that it is as "fair" as the three of you decide that it is. Put it on the table and see what happens. If someone thinks it is unfair, provided that you have good communication between you, it will come up. Watch body language and listen for unspoken hints that the others are uncomfortable, and if everyone expresses being OK with it and you're not getting conflicting messages from behavioral cues, smile, nod, and enjoy yourselves.
Said to me recently: "Look, I know you're the "voice of reason"... but dammit, I LIKE being unreasonable!!!!"
"Your mind is more interested in the challenge of becoming than the challenge of doing." Jon Benson, Bodybuilder/Trainer