eBike winter battery power My Bosch eBike has dual batteries which I usually ride with one battery.  I feel like as the weather drops to freezing the bike power has dropped off slightly and when I ride with the second battery added back on the power is raised back up.  Is this expected?  

Obviously I am aware that double the battery means double the range but I didn't think a double battery would provide more power than a single battery which was fully charged.
You're right - a cold battery cannot provide as much power as a room-temperature battery.   

I had an ebike based on SLA batteries, and in summer it would get 25 km range, which dropped to under 5 km in the cold frosty mornings of a winter.

It is possible to get battery heaters, common in places like Canada where its cold enough to freeze the water in a SLA battery and split the casing.  But on a bike you're going to be using battery power to keep the battery warm.  A better solution is to store the bike inside at both ends of your journey.

Perhaps a wrap made from a thermal blanket could help keep heat in, but there are downsides to that too.

Suggest you read To what extent should I insulate an electric bike battery during winter?