It does not matter what Windows reports. The two cards are essentially sharing video RAM, regardless of the configuration.
If you are drawing one frame, both cards have to have the exact same information loaded into them, thus lowering the available RAM on both cards to the same level.
If you are drawing every other frame, the same applies again.
You can look at it this way, if it will make you feel better. Yes, it is doubling the video RAM, but it takes twice as much data to render any given frame than it does with one video card.