We would have to make sure each house would have set powers. For example in the U.S., only the Senate confirms Presidential nominations.
Also, I would say that instead of picking a number like 15, (unless we do the upper house elected by the regions) we could say a percentage of all registered members (or all voters in the last election). If we did 1 for every 10, we would get either 11 (from the 111 people on the rolls) or 6 (from the 63 voters).
I guess we need to find out, how many Atlasians would participate in the lower house if it used a form of universalism.
That proportion thing is actually a great idea. I would say you base it on the number of voters in the general election.