They are both single chip cards so will have the whole 1792 to themselves. A dual chip card would split the ram and for memory purposes would be the same as two cards with 896 in SLI. Which would mean the total available would be efectivly 896.
The way it works is that a standard 260/275 has (14) 64MB [512 megabit] memory chips. When they step up to 1792 they use (14) 128MB [1024 megabit] memory chips. Same number of chips, but each each chip's capacity is doubled.