I don't know of any theme that pulls posts from two different blogs (installations of WordPress) or two different databases. But then, from the sound of what you're trying to do, that's not really the best solution for your problem.
The way I'd solve your problem (at least within the WordPress framework) is by using two categories, or perhaps custom fields. For example, have two categories on your blog (i.e. Pro and Con) and display posts from the Pro category on one side and from the Anti side on the other. (You could do the same thing with either tags or a custom field.)
If that doesn't quite make sense, my personal homepage at davidbhayes.com might actually help you see what I'm saying. That page aggregates the things I write at multiple blogs, puts them into categories (based on the blog they're from), and then uses the categories to determine what's displayed where.
I know of no publicly available theme that does this for you (far too small a market for someone to build one), but there are a number of tutorials about using categories to display multiple columns of content with WordPress.
I'm not sure that's the answer you're looking for, but I hope it helps.