I did this a lot. Both with wraps (used the SleepyWrap) and with soft structured carriers (Beco butterfly).
I used the SleepyWrap up until they were about 6 months old. I'd wear one in front, off-center, and then just carry the other on the opposite hip. Did this a lot at appointments (they were almost guaranteed to both be fussy and needed to be held and walked around) and when people came to visit me at home (because they were so afraid of strangers they both HAD to be held), and when I took them to friends' houses by myself. Ugh that reminds me of when I took them to a friend's house when they were 4 months old and they both SCREAMED the ENTIRE time. I was wearing one and holding one, and I had to go shut myself in the guest bedroom and try to calm them down. Eventually I just left. And they SCREAMED the entire drive home. Everyone felt so bad for me, and no one could help because of the stranger anxiety.
With the Beco, I'd wear one on my back and carry the other. Did this a lot for errands (like returning something to the Gap). Great for grocery shopping -one on my back, the other in the cart. I'd put the fussier twin on my back. DH and I both wear them on our backs when we go somewhere where we don't want to have to keep track of a double stroller, and it's fabulous!