At a 6 week ultrasound they found 2 yolk sacs 2 heart beats but only 1 heart rate?

I HAD A EARLY SCAN AT 6 WEEKS AS I HAD SOME BLEEDING AND THEY FOUND TWINS. ThEY PICKED UP 2 HEART BEATS BUT ONLY ONE HEART RATE AS THEY SAID IT WAS MAYBE TO EARLY TO PICK IT UP. bOTH YOLK SACS WERE DETECTED AND FETUS A AND B WERE MEASURED DOES THAT MEAN BOTH ARE OK OR MAYBE I HAVE LOST ONE. mY FIRST ANTE NATAL APPT ISNT UNTILLL IM 15 WEEKS, IM NOW 8 WEEKS.