Twin HBs with home Doppler.

Lisa
Early this morning I got up to pee and as I was getting into bed I felt a distinct roll or shift. It stopped me from getting all the way in bed. I couldn't see it but I put my hand down there and felt a large lump was on the left side. After a moment it went down again. 
Just now I listened to the HBs. They are both on the left now!! I couldnt hear anything but swishing on the right. I know I heard two distinct HBs because baby b has a faster one of 160ish and baby A is about 140. They were on top one another it seemed like. I didn't think this was possible because they have their own sac? Or can the still move around each other with the sacs? 
I'm 15+3 and I don't normally feel them unless I put my hand firmly on my belly and really concentrate.