Help Twin Ultrasound Question

I am 6w1d according to LMP but found out today I am having twins and are a little further back than I thought. Long story short, I am measuring 5 weeks (not sure how many days they didn’t say) but one gestational sac doesn’t have a yolk sac while the other does. Has anyone gone through this? My doctor doesn’t know if the one without the yolk sac will make it or I’ll just miscarry it. Any advice? Has anyone had a gestational sac measure smaller (further behind from the other)? I am just confused that’s all.