I know you are worried, but the truth is, there is nothing you can do. It is out of your hands. Unfortunately. But, there is a great chance that your baby's heartbeat will be just fine next time. It's early! Hang in there! Sending positive thoughts your way!! Welcome to the twins club, btw :)
My daughter had a low heart rate at around 8 weeks... they scared the hell out of me.... intimation that there was a possibility that the next time they looked it wouldn't be there... all that worrying and she's absolutely fine! I wouldn't worry about it so early on... I think heart rate is pretty subjective at this point in the pregnancy.