Good balanced piece.
As two great powers with one of the world's longest borders and a lot of shared strategic concerns in common adjacent areas, it's unremarkable that they cooperate in this truly modest fashion (6,000 troops). Since we want similar things for Central Asia, and aren't looking to sink more assets there, the U.S. should embrace the SCO, not fret about it like some pathetically paranoid superpower ("Are they plotting behind my back?").
As for "signals," both Russia and China say the same thing: we matter, our interests count, and we need to be considered part of any solution.
Absent the neocons' fantasies about "primacy," that's a beautiful world, assuming we remember that the Russians remain Russian and the Chinese remain Chinese.
Does this constitute a "great game"?