If a bookie allows you to place a parlay wager and he is also accepting parlay wagers on the opposite side it is fair game.
Many books will not allow you to make correlated parlays yet they will still allow you to parlay the 2 uncorrelated sides of the parlay. Is this fair that a book would selectively only allow you to make the 2 highly -EV side of a game that can be parlayed 4 ways but not the +EV sides? Rebate wager does this and several other books and I think it is nonsense, if you allow a parlay of dog/over you should honor a parlay of dog/under or none at all.