Yes, you can put both the fridge and furnace on top of each other in seperate compartments. Our trailer is exactly that way with just a simple 3/4 board between them.
You will need to cut two openings in the exterior wall. The top one, and largest, will be for the refrigerator. The bottom one, which is much smaller, is for the furnace. You can see both openings which have the outside covers on, in the following picture of our rig. Look just behind the front awning support and above our wooden name sign. The fridge cover has the horizontal openings and the furnace cover has the small square opening which is actually a round exhaust vent that I have a square wire cover on to keep mud daubers out. I have the same type of screening in the horizontal openings in the fridge cover. The other larger cover near the front of the trailer is for the water heater.
Here is a picture of the furnace opening inside showing a bit of the furnace and the duct that supplies the heat to the floor ducting. There is a screen that covers this opening and the fridge is sitting on a board just above the furnace and behind the front facing surface you see at the top of the picture.
It should not be a hard job to do except for possibly running and sizing the length of the furnace ducting. Of course, they also make small furnaces like those found in popup trailers that do not require any ducting.
Good luck and keep us posted on the progress please.
Barney
2004 Sunnybrook Titan 30FKS TT
Hensley "Arrow" 1400# hitch (Sold)
Not towing now.
Former tow vehicles were 2016 Ram 2500 CTD, 2002 Ford F250, 7.3 PSD, 1997 Ram 2500 5.9 gas engine