I didn't breastfeed twins, but I did breastfeed all 4 of my kids.
With my first daughter I felt let-down very strongly, I got painfully engorged, I sprayed milk out of the "idle" breast while DD was nursing...it was truly an experience.
I had NONE of that with my second DD. I did get engorged, but not nearly the extreme that I had the first time. Other than that...nothing.
The best indicator of how much milk you have is whether your boys are producing adequate poopy & wet diapers, and gaining weight. Sounds like they are doing well on weight? If you are concerned, I'm sure you pediatrician would have no problem with you stopping in for a weight check between appointments.
As far as increasing milk supply, one of the best things you can do is increase the number of nursing sessions. So make sure you are nursing as soon as one of your boys cues hunger (I know that clients of mine who had twins would either than nurse the second twin at the same time, or right after nursing the first, rather than waiting for the second to cue), and getting in a *minimum* of 8 of these sessions per day. If they are really young--as in less than about 4 weeks past their due date--you might want to even be aiming higher. It is absolutely normal for some newborns to nurse 12 times or more per day.
Beyond nursing, I can't imagine with twins that you'd really have much time left to pump. But if you do want to pump, you'll get more bang for your buck if you do it 30-60 minutes after finishing (or longer, but you might not want to get too close to the next feed) a feeding session than if you do it immediately after. Immediately after you are less likely to have any milk that you can pump out, what with nursing two, and there is a whole complicated thing with how the hormones work where you get more of a supply boost if you have two 20 minutes stimulation sessions as compared to one 40 minute session.
Good luck!
Jenn
www.babybodybirth.com
P.S. I was just reading through the other responses, and I've got to say, I COMPLETELY disagree with the comment saying that you don't need to worry about how much weight your babies gain. You *do* need to keep that in mind, because Failure to Thrive is not insignificant. A breastfed baby should actually move up in the weight charts during the first 4 months (like from 25th percentile at birth to 50th at 4 months of age), then start to slowly drop. This is because the weight charts are mainly based on formula fed babies, who have a different growth pattern. It really concerns me when a mom is told that a 3 oz weight gain per week is fine in a 4 week old. A weight gain that low (it should be more like 8-12 oz!) is a sign of a low milk supply that should not be ignored.