Im 7.4 weeks with twins and baby b heart beat itsnt so well will it get better is this common in twins?

It is not uncommon for the heartbeat of twins to be misinterpreted or hard to interpret on an ultrasound. Baby b could simply be slower at developing. It is not uncommon for twins to be born with one baby being quite a bit larger than the second baby.

Here is a case of a similar situation

http://en.allexperts.com/q/ObGyn-Pregnancy-issues-1007/heartbeat-4.htm

And for inspiration

http://www.timesonline.co.uk/tol/news/world/article1738812.ece

I wish you all the best.