It sounds like my summers and winters are very similiar to yours -- I live in Minnesota. I just got my ball and setup a few months ago, so I haven't had to go through a winter yet. However, I do have mine set up with a uth on both sides. I am using 11" flexwatt cut to size for each side (so there probably is only about 5 inches of floor space in the middle that isn't heated).

I am using a herpstatII (I know it's a lot of money, not saying that's what you need, that's just what I have). I have one side set at 80, and the other 92 (and my ambients usually don't fall below 75).

You could go with one uth, and then supplement it with a night lamp or cht in the winter, but then you risk sucking all of the humidity out of the cage.

Hope this helps,

George