I use two ... the v-shaped pillow with ordinary pillows/cushions underneath or the Eze-2 nurse pillow which is a blow up thing which hooks around your waist and is slightly contoured so the babies lie nicely and don't roll off. Both good.
I used a firm foam, semi-circular one that curved around me and kept them from rolling about too much, when they were smaller I put normal cushions under it to make them the right height. I found it much better than a bean bag one. Good luck, always happy to hear about breast feeding twins as I didn't get much encouragement from the hospital and would have given up if I hadn't already fed my daughter!
Everyone in my Twins club who did manage to breastfeed did it with the help of this BRILLIANT feeding pillow: Our Health Visitor told us about it even tho' she's not supposed to do "product placement", but it's really so good, I can't recommend it highly enough. Look on eBay to see if you can find a second hand one. £55 is a lot of money, but if it enables you to breastfeed you'll save that amount over and over! Good luck, breastfeeding twins is a challenge, but pretty darn wonderful if you can manage it! http://www.twinsuk.co.uk/product.php?id=100699