hi, i had that with this pregnancy. at 6 weeks there were 2 sacs 1 with a baby and a heart beat and the other was empty but very close in size.
2 sacs usually means fraternal twins.
sometimes 1 just goes away, or the other sac can form a baby upto 14 days different in age. like with me i went for another u/s at 8 weeks and there was only 1 baby.
the heart beat usually is visable at about 6 weeks.
good luck