Doesn't really sound significant.
It shouldn't be anyway. You're already running double buffered, maybe even tripled buffered, which causes frame lag anyway.
A card twice as fast as to render a frame ahead (2 frames).
Two cards would each render 1 frame, in double the time per frame. Should work out to be the same amount of input lag.
(I could be wrong about this)
In a 60fps game, you already have 50ms of lag with triple buffering. 33.33ms of lag with double.
In a single buffered game, you still have 16.67ms of lag, since you have to wait for the frame to be rendered before it gets displayed. The only way to avoid lag, would be to do immediate rendering, in which case you would see things being drawn onto the screen. (and no conventional display device would support it anyway)
In a 30fps game, your lag would double.
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