I found it easier to sling one baby & carry the other in arms when mine were newborns, once they were old enough for the hip carry (which was around 4 months for my kids, granted that required the top rail of the sling to be very high & very snug on their back/neck) I carried one on each hip in ring slings or pouches (or one of each). Just put one sling on, put baby in it on one hip, then put the other sling on (other hip, obviously) & put the other baby in. The 1st sling tends to slip off the shoulder cap when you're putting the 2nd one on, alot of the time I was in a hurry (two frantic babies, it had been at least 10 minutes since they'd nursed & that was simply unacceptable) so I did end up w/ at least one sling rubbing on my neck alot of the time (still better than screaming babies or trying to juggle both in arms or not being able to go anywhere/do anything cuz they nursed 24/7), but if babies are content to give you a few extra seconds it's easy enough to lift baby's & sling enough to take the tension off teh sling & then reposition the shoulder as needed.
My girls weigh around 27 lbs at 29 months, I stopped double slinging (well, pretty much any slinging) awhile ago (I don't even remember when . . . last fall maybe?) when I decided I didn't want them nursing when we were out running errands & such. Being in the slings (hip carry) just puts them right THERE & makes them want to nurse all the time. I did end up pulling out my solarveil ring sling & carrying Lexie in it for awhile at Sesame Place this week & it was still quite comfortable to carry her in it, I suspect that I could still easily sling them both in the hip carry (granted not much arm mobility w/ a kid under each arm, though at this age one or both could be slid around toward the back (behind my shoulder) if needed) at least for short spurts, I just haven't had the need. They usually walk, most of our shopping is in stores w/ double carts so they ride in them, I *finally* have a double stroller that I like & they actually like to ride in it, so that works for other stores . . . they're indepenant kids so there's not much need to slings anymore.
I will also second the recommendation (at least once they're older) for at least one, if not 2 ABCs. I got my Kozy when the girls were about 9 mo old & have LOVED it for back carries. I used a ring sling or pouch in front (put the ring sling/pouch on, then put baby in ABC on back, then put baby in ring sling/pouch on front (tummy-to-tummy), or would use the Kozy when I had a single stroller or a single cart or was carrying one baby in arms for whatever reason.
The front/back combo would have been useless for me much earlier than that because my girls nursed so often/much that I needed the hip carry combo where both could nurse as needed, but once they were old enough to not nurse every 5 minutes, I've LOVED my Kozy (& more recently I got a Pakka from Boopa Rapture as well) for back carries. I did just recently try using both ABCs, one in front, one in back. I found it awkward in that, as far as I could tell, you couldn't get the front baby (or in my case toddler) out w/o also untying the back (it may have been worse because my Kozy has extra long straps in case dh wants to wear it so I had everything tied in front & both have black straps so figuring out which straps were which was a nightmare. I'm sure it would get easier w/ practice, but having tried it, granted only for a few minutes here at home, I think w/ younger babies I'd still perfer using a pouch or ring sling (no advantage of a ring sling over a pouch in this application because the adjustability of the rings is lmostly lost w/ the ABC's straps coming over it, I prefer a pouch, but if you don't have one, the ring sling will work) in front when using the ABC in back.www.thebabywearer.com
has a twins & multiples forum you might check out. There are people on it who have used the slingset mentioned above & I believe there's another special "twin sling" that is currently being tested (I'm moderator there but w/ my two beyond must sling use I don't pay real close attention to the new products coming out, other than to read from a moderator perspective). There have also been some recent (w/in the last month or two, but it's not a horribly active board) pictures of mamas slinging 2 kids (twins or different ages) using 2 wraps & such to give you other options to try as well.