Twin A measuring 8 wks twin B measures 5wks

Hi! i’m looking for some help/advice. we had our first ultrasound today expecting to be 11wks and found one baby measuring 8wks (we knew my dates could be off due to irregular cycles) with a 174 heartbeat. then after looking around a bit the doctor found another, much smaller, sac behind the first. doctor said it was hard to see much, and was unable to find a heartbeat. the second sac measured around 5wks. she said it was unlikely that baby B was viable but set up another ultrasound in 2 weeks to be sure. i’m wondering if anyone has had experience with this? is it possible for one twin to just be smaller than the other and not have a heartbeat yet? could really use some help! thank you🙏🏻