Congrats on the twins! Mine are two and they are a ton of work but SO much fun!
At 20 weeks, your twins should be pretty close to being the exact same size but you'll probably start getting a ultrasounds scheduled pretty regularly. (They'll be checking to make sure the babies are developing at about the same rate from here on out.) Twins normally arrive at the 36 week mark. Mine were 6.0 lbs, and 5.9 lbs.
The Graco Duo Glide Stroller is GREAT! I also really liked my twin nursing pillow (you can also bottle feed). Check out www.doubleblessings.com for twin products.
I didn't need anything else except for two swings and a TON of burp cloths, blankets, and newborn onsies. The first six weeks are the hardest - because you're sleep deprived when you're caring for ONE newborn . . . you get practically NO sleep during that first six week period but by having a ton of burp cloths, blankets and onsies you can cut down on the number of loads you have to run through the washer! Twins actually get easier at about the six month mark than singletons because they start getting interested in each other. I got more sleep with my twins than with my oldest because they would chatter at each other in the morning rather than yell for me as soon as they woke up.