In the second trimester, too, the movement can be really inconsistent even with a singleton. The baby is still small enough to have lots of freedom of movement, and maybe even if he/she is moving, you can't feel it because of where the movement is happening. But even later on, I always felt DS move more than DD2. I think it was because of their positioning, because as quiet as DD was in utero, she's definitely the more active one now, and was the healthier one at birth. I remember being all in a panic one night, too, because I hadn't felt DD all day. Anyway, it turns out that what I thought was DS was actually DD, because DS had moved.
Yeah. I agree with the PP-- it can shred your nerves, having two babies inside of you. Nothing like carrying multiples to teach you a little faith.
Past 30 weeks, or maybe 28, if I felt no movement at all for most of a day, or I felt what I was fairly sure was movement from only one twin for most of a day, I'd seek out an ultrasound to check on things. But before then, movement is going to be inconsistent anyway, so I would try really hard not to let it make me crazy.