Possible Twins!

Hi, this is my 4th pregnancy and I went in Monday for a follow-up ultrasound for bleeding and mild cramping. Dr. saw 2 sacs, one with a heartbeat and the other with a possible fetal pole. I was measuring exactly 6weeks. I go back for ultrasound at 8weeks to check for heartbeats. 
I was wondering if anyone had any success stories with both babies being viable? Or if it could be vanishing twin syndrome?