It doesn't have an impact on the viable embryo if they are fraternal. If they are identical, it can sometimes be a problem (there is something called "twin to twin transfusion syndrome" if they are closely linked). But it sounds like you are talking about fraternal twins, and in that case, the one sac will just be there for a while on your ultrasounds and ultimately either be reabsorbed by your body or pass out when the other baby is born. I had twins to begin and then a loss of one at 8 weeks pregnant, and though we could see both in the ultrasound for several ultrasounds after that, ultimately we only had the one (healthy, full-term) baby and the presence of the other small embryo did not affect it. (It was just a little sad to see it those few times.) It's called "vanishing twin" syndrome, you could google it and read up on it.
This actually happens more often then you might think. I know more then one member who had the same situation. I don't believe it should have an impact on the viable fetus, but of course you should talk to your Dr. He/she is the best person to reassure you. Good luck to you and congrats on your pregnancy.
You have 2 saks and one heartbeat? Are you having twins?
My cousin was pregnant with fraternal twins and miscarried one. The other was fine and she carried it full-term. You should call your Dr and ask them what you should expect.