Doctor found yolk sac gestational sac and heartbeat on one twin but the other twin did not show a developed yolk sac or heartbeat Is that okay?

You did not indicate what week you were at in your question. I am at six weeks one day today, and went to my doctor today for an ultrasound. I had one twin with a fetal pole and a heartbeat that was about four times the size of the other one, which had the gestational and yolk sac but no detectable heartbeat. My doc said that there is a possibility that the smaller one will not be viable (that it may have arrested), but that a followup ultrasound in two weeks is necessary to see if there is growth. The ultrasound tech said that she has seen fraternal twins implant at different times, thus resulting in different size sacs and different rates of growth. Assuming you are not far along, I don't think the size itself or lack of a heartbeat is definitive of having lost one of them, and they will probably be able to tell at a repeat U/S. You can also look up the concept of a vanishing twin - it is apparently a common phenomenon. In most instances, it is not harmful to the remaining twin when it occurs early on.