Hmm, I guess it depends on what your specific opposite theme is. E.g If it's two different personality types, or just the fact that you like something everyone else hates. Could be something about living on the opposite side of the world - in which case I'd use a travel kit. Maybe a sibling kit would have some wordart in it you could use? I imagine any kit could be used for an opposite page.
Found this one
You could even use two totally separate kits. Split your page. Use one kit for one person, and a different kit for the other person, depending on what thing they're opposite about.