We did, back when the sites first went into beta. Ask Ubuntu was opposed, so we stayed separate:
http://blog.stackoverflow.com/images/wordpress/ubuntu-vs-unix-graph.png
You can read http://meta.unix.stackexchange.com/q/93/73What is the relationship between Linux/Unix SE and Ubuntu SE to see how it works since we stayed separate. Also http://meta.unix.stackexchange.com/questions/990/how-should-questions-cross-posted-on-ask-ubuntu-be-handledHow should questions cross-posted on Ask Ubuntu be handled?, which mentions cross-posting and why we prefer you not do it. At this point it's entirely too late to merge the sites, plus Ask Ubuntu is a special case because they're officially endorsed by Canonical, and I'm sure Canonical wants their official help site to be Ubuntu-only.