If your org. uses Roberts Rules as your parliamentary authority, then your PTO can create a standing rule before elections or even the night of the elections stating that members of the same household cannot hold office concurrently.
You could get more specific and state that the office of president and treasurer cannot be held by members of the same family in the same school year.
When you have time to amend your bylaws, I'd suggest putting it in there.

we have our nominations coming up in May. There seems to be a lot of problems this year because there are 2 people going for the same position both with the PTO President and the Treasurer. My concern is that husband and wife are going for president and treasurer. I believe this could be a conflict and is wrong. Is there anything written or does anyone know anything about this being ok or not? Our by-laws don't say anything, but at the end it does say that we use Roberts Rules.