This is a subject I'm rather excited about, as I'm starting to nurse them together out in public more and more.
First of all, I feel good showing others that it happens. This is the activist part of me thinking, "Hmm. Maybe this will get one more mom out there knowing it can be done if she gets pregnant with twins."
Secondly, I never open my shirt, I pull it up from the bottom so that I don't feel so exposed. The babies' bodies cover most of my tummy. I put them both on my lap with their bodies facing each other, latch one on, hold up my shirt with my teeth while I latch the other on, then let go of the shirt so it falls between their bodies without blocking their faces (or at least their eyes). This is easier while sitting on the floor and I'm getting pretty fast.
Another GREAT thing I've discovered is using my sling while NOT wearing them in it. This time around, I bought a sling with a looooong tail and no bars or plastic stuff, just the metal ring. (My last sling was an over the shoulder and the tail was a thin, useless thing, smile...)
THIS time, I use this tail for all it's worth. Often, I'll wear it around my neck just to use the extra cloth as a blanket to cover up where I might feel exposed but to not actually have to hold onto it...also, I've discovered several ways to nurse while walking in it, which I could never get the hang of before but which has proven to be a godsend while out with both babies. My babies, bless them, are usually not fussy at the same time.
Another helpful use of the sling is to nurse one in it in the cradle position which frees up your hands a bit. Then, I lift up my shirt on the other side and nurse the second baby on that one.
Practice makes perfect. I suggest spending time at environments like La Leche League meetings where you can let it all hang out and get the hang of doing it in front of others, and then you can work out the details like discretion if that's important to you.