Worried about small twin

My ultrasound at 6 1/2 weeks revealed two babies after IVF, transferring two embryos. The first baby measured 7 weeks and a day and had a heart beat of 160 beats a minute. The second was measured at 6 weeks and a day but the nurse couldn't really find the heartbeat. I am so worried about the second baby. Now that I know there are two I want them both to survive and be strong. I have another ultrasound scheduled on Monday, I'm very nervous about it. Has anyone else had this kind of experience? Is it normal for twins to grow at different rates?