I see these usually installed on the drivers side, between the cam cover breather, and the intake. Under normal driving, I think this is actually an intake to the crankcase breather system. The suction side is on the intake plenum...hose goes to the PCV valve in the passenger side of the cam cover. I would think that a catch can on this side, would be as beneficial. Under boost, the other tube will become an exhaust, due to blow by, so catching oil over there, before it gets back to your intake (if that's how you have it plumbed) would also be a good thing.

In short, I think you need two of these cans.