Hi Helen! I really don't know too much about slings in particular, but I know it is possible to carry twins in one wrap carrier. I have a Moby wrap, and the booklet has a couple pictures and directions on carrying twins. The Moby is quite similar to a hug-a-bub, but doesn't have the pocket, and is cheaper. I can breast-feed Emily with her in the wrap. I don't know about doing two at the same time, but one certainly works.
From the pictures in the instructional booklet, it looks reasonably comfortable to carry smaller twins in one carrier, but once they get a little bigger, it looks a little awkward. Too much baby on one person! Maybe get two wraps, and then you can wear them together when they are small, and you and DH can each wear one when they get bigger.
BTW, I love my wrap. It is my extra pair of hands! Very comfortable, and easy on the back. In fact, I think shortly after delivery, my back felt much better and supported when I had the wrap on!
Anyways, all the best with your decision!