Well actually SL viewers DO take advantage of a multicore cpu, though the main process is not multitreaded viewers can run secondary process and treads on several core. The improvement is not epic but tangible.
As been told in countless other posts any multi gpu/card solution, sli or crossfire, doesn't offer any real benefit, for some reason historically viewer hated SLI solutions and it seems things are not going to change. Advantages will be nonexistent at best, it will cause total instability and crashes in worst cases.
My advice for a cool SL system.
A modern quad core cpu, nothing extreme you don't need it.
A good single gpu card, more powerful the better.
A fast hard disk will help too, sl rely a lot on local cache, ssd would be ideal.
Then it depends on what kind of experience you seek in SL, anyway, if you are running two viewers on the same monitor this will not impact processing power for a single session will be on foreground every time. In this case memory become the only critical point, both vga card memory and system ram memory (though six/eight gigabyte of ram will allow you to run almost everything).
P.S.
DX11 support is totally irrelevant. SL have nothing to do with Microsoft DirectX technlogy, it's based on openGL (luckly for me or it would not run under linux).