I do not think there is anything in the rules against that. However, if you are up on the house with BetPhoenix, then try to get a freeplay from Wagerchief, chances are you will not get it. Especially if you cash out from one and then try to immediately get a bonus from the other.
What you could try to do though is to sign up to both at the same time, keep one book for nfl and the other book for whatever else you want to bet on. They might buy that.