I have not tried this, but Barkeep let's you split an action bar into as many pieces as you like, and splatter those pieces all over your screen. So if you need to "borrow" 2 buttons from Bar2, you can still use the other 10 somewhere else.
Edit: Err, no it doesn't, sorry. I just got a good look at what it's doing. It's some nice trickery, but it's still wastes buttons from incomplete bars.