“If I use a 200-300wt wind resistant fleece, what temperature range should I expect the fleece sleeping bag to go down to with two people inside?”

There are a LOT of variables here. So here’s just me and my wife. We would be comfortable down to about 60F.
Here’s a warmer, lighter and more compact option:

My wife and I use one Western Mountaineering Megalite opened up fully. She then cut out some nylon blanket (similar to 100wt fleece) in a pie shape and sewed on WM zippers. We now have a Sweetie Pie. On hot nights, the nylon blanket is on top. On cold nights the Megalite is on top.

The Megalite is 24oz. The pie shape blanket is 10oz. We now have a ‘couples’ bag that keeps us warm to about 35F and weighs 34oz. I carry the bag, she carries the blanket.