What do you think about overwintering a one deep hive on top of another one that is stronger?
How to separate the two hives? Will a simple wire screen that will not let the bees move from a hive to another enough?
I would use a double screen set up. 1/4 plywood may create condensation problems for the bottom hive. It will not let heat rise unrestricted. I would have the hive entrances rotated 180 degrees from each other to help with bees drifting to the other hive
Actually you shouldn't be doing inspection in the winter. As for heating for 2 hives. In theory the stronger bottom hive will generate the same amount of heat they don't know the other cluster is above them. Heat rises so excess heat will drift up and help keep the smaller cluster warm. It sounds good in theory, however, I don't recall any scientific studies that supports it makes a measurable amount of difference
if you have none I would suggest you build a set of double screens. you don't use them a lot but when you need one they are extremely handy. you can use either wire window screen or 1/8" galvanize wire for the screen material. after a bit of use a hive will propolis the window screen so it will at least appear somewhat solid (but none the less should allow heat thru). <at least one fellow I know use the propolis 'window screen' version of these for cloake boards to produce large number of queen cells in the spring time.