I'm going to try this from different points:
First, as long as this man has a child in the school, it shouldn't matter who his friends are.
Second, who has time and energy to be both Site Council AND PTO pres...lol...? I think there are some people in this world who are truly amazing people with one thing in mind: to do the best for children. I think as long as this gentleman is looking out for ALL children, and he somehow has the time and energy to do both, and it isn't going against any bylaws, More Power To Him. If it goes against the bylaws, well then, let him know that he can't do both. I don't think he should anyway, but that is up to what is in your bylaws.
Third, I get the feeling that there may be an affair, or some sort of outside relationship, going on. On this, and this alone, I might have issues (for both of them). If there is any hanky panky going on, I would have trouble simply because in a town as small as mine, everyone knows who is sleeping with whom. I think that (MichelleB- back me up here...lol...) it is up to the adults in charge, whether it be a principal or PTO leader, Site Council pres, or whomever, to conduct themselves as examples to the children they serve. If they're just dating and not doin' the dirty, I say "Lord, I hope love blooms and grows". Have them over to dinner at your house and get to know them. It might be true love and not just some dirty dancing...
Fourth, use this guy's ideas, pick his brain on how to build your group. What kind of group did they have before at the old school? Find out from them both. I'd love to have the pres and principal from a different school bring all their ideas from another school (only if I had to do without our current principal... wouldn't give her up for anything.. but if I had to...)! Shoot, they've already done it and worked all the bugs out at another school! PICK THEIR BRAINS!!!
Fifth, Still feeling wary? Call up the old school PTO leader and ask. Was this an ugly situation where they had to leave? Or were they sorry as hell to see them go? It isn't spying, it is good business sense.
Good Luck!