Incorrect. The memory is mirrored on both cards so they are using local copies of the same data.
"You can then reasonably call it 4GB of VRAM"
No you cannot. VRAM is not shared, only one cards VRAM is used.
This is the main reason why multicard setups are not cost effective, you pay quite a bit for something you are never going to use.
Ah yes, my bad, should have wrote total VRAM amount is used. You are correct of course! They are mirrored.