260 in sli. no gain in performance? why not?

Charts, benchmarks Gaming Graphics Charts Q3/2008, 3DMark06 v1.1.0 3DMark Score

how come running two 260’s on sli, show almost no difference, than a single 260?

Because, at the resolution they’re showing (1280x1024) the resolution isn’t high enough to call for additional horsepower. SLI typically shows gains at resolutions above 1600 x 1200.