Congratulations! I, too, am a mother of b/g twins. They are 18 months old and we've had so much fun!
I have been a hostess of many showers and I will tell you it's a lot of work and it's very expensive. A small gift is appreciated. It let's the hostess(es) know you appreciate our efforts so much that, although you couldn't do much (expecting more children is expensive), you wanted to do something to let us know. A few gifts I've received have been: a guest towel with my initial on it (readily available at most boutiques), a scented candle (you can use an online 20% coupon at Bed Bath & Beyond or Linens n' Things, etc), a jeweled picture frame (Hobby Lobby 50% off), a small pewter serving tray, a box of kitchen matches in a cool iron box, or a decorative cross just to name a few. I've loved everything I've ever gotten! It doesn't have to be expensive (we'd actually prefer it not be). It just needs to be special; something from you that when we see it, will remind us that we gave you a shower and you appreciated it very much. My heart smiles when I see that frame or refil the iron box with my kitchen matches.
As Guest of Honor, for my hostesses, I've bought some of the things mentioned above and I've always been quick to send a follow-up thank you note to the hostess(es) for throwing the shower. For my first baby shower hostesses, I found some cute "Limoges" style boxes that looked like pacifiers at Tuesday Morning. They were precious and my hostesses went crazy over them. They were inexpensive but so fitting for the occassion. Whatever you do, put some thought into it and make it special.
Enjoy those babies!