One thing you have to think about it that the further along you get the heartbeats will drop until about 17 weeks or so when they become more stable.
Our twins are both boys. Their heartbeats were about 175 during 8 or 9 weeks. at 13 weeks they were both 153. At about 16 weeks they were at 145ish and that is what they remain now.
Hi! My babies' heartbeats were in the 170's around 8-9 weeks as well, but they're both in the 150's now and almost every time I've had an ultrasound (once a week since 5 weeks), their heartbeats have been the same as eachother's. We are having one boy and one girl.