There is a bug in that a 2AP attack displays a 1AP cost on mouse-over. I wasn't really paying attention when dual-wielding, but I know that if you're using a 2-handed weapon it will do it. It is intended to cost 2AP, though.
Basically, for every hand you have something equipped in, there's a 1 AP cost. So, the only way to get an actual 1 AP attack is to have one hand empty.