The engine has to be specifically written to take advantage of SLI, and Poser is not.  I know the marketing info for various SLI cards/motherboards sounds like it works everywhere for everything, but people have benchmarked preview performance in various apps and it doesn't improve performance unless the app is written for it.

The best way by far to boost your preview performance is use one of the alternate display modes, particularly Outline - since the card doesn't have to render any polygons or lighting or textures it is muuuuuuch faster, although less comfortable to work with than the other modes.

edit: More generally, just don't load things that aren't necessary for the given task.  If you're working on a pose, you don't need to load anything but the character (without textures) and maybe the prop(s) that the character is interacting with, if any.  Don't load textures until you really need them, don't load every single morph target if you're only using 10 of them, etc.

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