Anxious!

Found a couple of weeks ago I'm having twins! I'm 6w1d today. My last ultrasound was at 5w4d. There was a heart beat for baby a, but they said baby b was measuring a few days behind and had no heartbeat yet. Is this normal? should I be worried that baby b won't be viable? I am high risk due to medical issues (even if it was a single) so I get checked weekly. I go back on Tuesday (11/7) for another blood draw and scan. Also, my HCG is very high, and keeps rising. My Dr told me it would slow down and steady out... anyone else have this happen? What was your hcg at 6weeks? Sorry...this is all new to me and I have a million questions LOL!