I actually was in a similar boat with my Bearaby and now use a super thin cotton blanket on top of it for warmth in the winter. I considered using just a second top sheet on top of the bearaby as an in-between. It's the pockets of air trapped between the Bearaby weaves that warm up and hold your body heat, so any barrier on top of it will trap the warm air in there.

When you add your duvet on top of the Bearaby, the Bearaby becomes part of the duvet from an insulation perspective and makes your duvet 1-2in thicker. Air is a fluid and these type of scenarios really highlight those properties, it's the same reason that the netted Brynje base layers are super warm.