1) Will AE use 1.5 or 3g of gddr?
Never. Having two cards does not double the memory because each card will redundantly store data in its local RAM.
2) Will AE use 192 or 392 cores?
AE doesn't care. It will use whatever the driver makes available and that is an either/ or thing. It's all or nothing. If your driver enables both cards all 392 cores will be avialable, if foer some reason the driver goes belly up, it may only show one or none at all...
3) Do I have to install the SLI bridge?
For CUDA you shouldn't have to, as it's managed directly over the PCI bus.