I agree there's a slight difference in focus, but they're close enough that I think separating them into separate sites diminishes their audiences.
Just a personal take: I'm browsing codereview.stackexchange.com today, but I'll probably rarely come back unless I have a really specific question that I somehow feel would get better answers here. I'm an active StackOverflowe user but I don't have time to increase the # of stackexchange sites I look at.