Different sacs, same placenta

So my doctor is thinking our twins have two different sacs but could be sharing a placenta. I’m going to a high risk OB on Tuesday for a 4d ultrasound for sure. Of course my OB is telling me not to worry...it’s not exactly uncommon but they will have to keep a closer eye on the babies if they are sharing.

I’m 13 weeks today and the babies are looking great & growing at the perfect rate so far but of course I’m freaking out a bit.

Has anyone experienced having twins with different sacs but the same placenta? Any info or experience you’ve had would be great to know.