Serious Question

So.... I found out three weeks ago that I had two sacs on an ultrasound, my doctor then told me because they were both empty that I’d most likely have a miscarriage, they compared my HCG results from my first and second visit and they were rising but not doubling, fast forward to my third visit and I had and ultrasound done, by that time I had spotted and started to heavy bleed with clots, the doctor told me one of my sacs was collapsed and that I had passed a majority of it but the second sac, looked so much fuller and didn’t have a weird shape to it like the week before , she wants me to go back Monday for another ultrasound to see if I’ve passed the second sac because she still didn’t see anything in that one, not to mention I’ve been measuring at 5 weeks for the past 3 weeks when I was suppose to be 8 weeks at my first visit.. so I want to ask has this ever happened to anyone and the second sac turned out to be viable? And how common is it to have two sacs that are blighted ovums with a twin pregnancy?