"Always make the cage as big as possible, but your assertion that a pair in a 8x3x2 cage is similiar to a single monitor in a 4x3x2 (corrected your math) is not realistic. They will not draw a line and keep to sides, they both will use all available space, as long as their aren't issues with the other lizard."
No they wont draw a line between the 2 sides, but you still have 2 possibly same size animals in the same amount of space, so basking spots, hides, etc etc are relative size, being cut in half is relavent as 2 animals are there not one. Example try this at home, step into your bathroom and stay for a few hours alone, then with another individual the same or similar sized to you, see how comfortable the 2 of you are, even with your wife its difficult let alone for your whole life in that room.
"If there are issues, being twice or three or eight times as large may not be enough anyway"
I completely agree with that, I remember a suggestion to properly keeping a trio of adult Albigs safely but still under good observation for their safety (due to their cannibalistic tendancies) as a cage 40x23x8 meters, but how much room can we provide, if they get along then good but if they dont, any way you do your math 2 cages are needed.