Ah, if you're talking about bonuses to audio and visual perception... they're different bonuses, and not so much stackable as interlockable. They matter when the perception is going to be of a certain type - audio perception helps you hear the crack of a twig... but does nothing to help you notice that well concealed gun turret. On the other hand, Visual perception is great for the gun turret, but you might completely miss the sound of someone breathing. Put them together, and they'll work nicely to make a generalized bonus to the dice pool, with some few exceptions, like touch or smell (olfactory boosters can do wonders with smell as well, while touch sensitivity might be duplicated through an adept power.)
You're not going to see +2 to audio and +2 to visual = +4, but you'll see the +2 bonus come into play more frequently.