Holy poop! Good luck!
Honestly the fact that they have heartbeats is good, though I know anything could happen.
I was a surrogate for my best friend and they transferred 6 embryo's. Of course, they did all 6 because the quality wasn't super and they didn't think they'd survive a freeze. Anyway, 3 stuck and over the weeks, we saw one heartbeat and 2 with none. One sac had obviously stopped growing at one ultrasound and then a week or two later, we saw the one heartbeat and none in the other. Another week and still no HB in the other. Long story short, I ended up carrying one baby-the others just stopped growing and were reabsorbed.
My point is, we didn't even get to the heartbeat stage and were told once there was a heartbeat, the chances of loss went wayyyy down. I'd assume in our case part of that was the quality of the embryo's, but still.
It does sound like the one might be having a hard time? How low is the heart rate? I don't know anything at all about having a large yoke sac, so I'm no help there, but my 2nd baby's heart rate was very low the entire time and she was fine. I think she was just super laid back...even her blood flow. haha. (This was the kind of baby that I could go a week without feeling a single movement in the 3rd trimester!) I'm not saying nothing is wrong because at this point, waiting is the only way to know for sure-but it could be fine
Good luck and keep us updated!