Covens are chosen randomly. I try and make it even, so that one coven never has more than 3 members (I dont mean active... I am sorry if one ends up with 20 active members while the other has 2... thats out of my control) than the other. You can drop hints, but I am not guarenting anything is going to happen in your favor. I mean, some people will have to be the good guys, others the bad!
so good luck! Choosings are done about twice a week.