Sorry, I remember seeing this thread and wanted to come back to it but forgot.

You're correct, when cards are SLI'd their onboard memory is not added together and it is shared (duplicated) between them.
The GTX 590 has always been marketed with 3GB of memory but it's really 1.5GB + 1.5GB which actually can't be added
together.

I don't know if you can utilize the second GPU at all if you disable SLI on the card itself. Maybe the 2nd GPU is connected to
the second DVI port (I have no idea). If it is then you can create game profiles within InnerSpace's configuration to take
advantage of different monitors in which you plug into different GPUs. If you are able to use the GPUs separately they're
each an underclocked GTX 580 when not SLI'd.

Dual GPU cards are a headache and they're never recommended to use with multiboxing.