Ultrasounds questions - 16 weeks and anxious!

With my first pregnancy I started seeing my OB at 6 weeks with appointments every 4 weeks after that. They used ultrasound at every appointment to check the heart rate and measure baby. My last OB was in Northern Virginia.

We moved to Upstate New York this past April and found out we were expecting in November. My new OB wouldn't see me until I was 10 weeks. At that appointment we found out we were expecting di/di twins.  She took me they'd start monitoring closely because it was twins. At my 14 week appointment they did weight, blood pressure, used a Doppler to attempt to find the heart rates and sent me on my way. 

My next Ob appointment was scheduled for 20 weeks,  but my maternal fetal appointment is the week after. I called to move it until after maternal fetal.

I'm super anxious,  they had a hard time finding the second heart beat with Doppler and I'm unable to find it using the one i purchased.  Last ultrasound was at 10 weeks,  I'm at 16 weeks now and won't have an ultrasound until 20.5 weeks.

I hate not knowing if everything is all right. :-( debating if i should consider seeing a different OB.