Graphic's card memory

I had a Zotac GTX 570 Amp edition graphics card in my PC. When running games, GPU-Z would show that the card was using all 1280 MB of the VRAM. I've just purchased another of the same card and am now running them in SLI.

The problem is now GPU-Z says each card is using only 386 MB of video memory (for a total of 772 MB). As far as I know, you don't get the benefit of the ram on the 2nd card when running in SLI. But why would my overall available memory actually drop by almost half?

I have enabled SLI in the control panel and both cards are running at their normal core clock when I'm using graphics intensive programs.

Does anyone have any suggestions or know of any settings I need to adjust to get the default 1280 MB back?

Here is what I currently have:

(CPU) Intel i5-2500K

(Motherboard) Asus P8P67Pro B3 Revision

(Memory) 4 x 4GB Corsair Vengeance DDR3 memory

(GPU) 2 x Zotac Geforce GTX 570 Amp edition 780 MHz Core Clock

(Storage) 2 x 120GB OCZ Agility 3

(PSU) Corsair TX750 Enthusiast Series V2

Any help would be greatly appreciated.