The characters aren't actually circular bases that are 1" in diameter. The characters are actually people-shaped, with appendages and postures that can only vaguely be approximated as cylinders. If you have two individuals, standing back-to-back with no space between them, then this would be represented on the grid as:
XXX
ABX
XXX
But it could also be represented as:
AXX
XBX
XXX
The difference between these two situations is only the arbitrary orientation of the grid. But the grid isn't real, within the game world! The grid is only a tool to help us visualize what's actually going on. Since the in-game reality is identical between these two situations, we are obligated to treat them identically within the rules. If you wouldn't allow someone to pass between these characters in the first orientation, then you shouldn't allow someone to pass between them in the second orientation, or else you're being inconsistent.