sli video card question....

i was wondering for sli video cards if one video card has a total of 512mb video memory size and the other is the exact same one would they total up to 1gb video memory size, or would they still both be running at 512mb video memory size in sli mode.
 
Got a feeling that you cannot just "add up" the memory, as there will be duplication between them - not sure if an SLI pair gains any memory advantage at all over a single.
 
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