I believe SLI works only when it's the same type of card. I know that's the case with my 2 560's.
Some cards do support "mixed SLI", but only for minor revisions within the same family. The 570 and 580 could probably do SLI together for example. But a 780 and 295, forget it.
Plus, you could probably keep a hot air balloon aloft with what you'll have blowing out the back of a machine with both of those cards running full blast.
when you want 1 render context you need 2 exactly equal GPUs
,but if u want to use 2 render contexts or want to render on one card and do calculations on the other you can have any 2 GPUs you want(and they needn't be connected by a SLI bridge).
I think you can't share or exchange data on both cards if they aren't connected with SLI or Crossfire.
This is why I asked about multiple contexts in the first place. I know the GPUs are different, I just wonder if I can render with them all in parallel by manually dividing the workload between them, and send the data over an SLI-bridge to avoid a PCI/CPU bottleneck.