from Dan Powell:
Some AU plugins have a side-chain input for things like compressors and so on. I'm not sure what you mean by whether they do a good job of it, but it seems it's the only way to get a second audio input to a plugin.
A lot of my own Max patches are experiments in cross-modulations between two audio signals (FFT comparisons and so on). So my long-term goal is to be able to build these as AU plugs.
A very simple two-input plug would be a Ring Modulator, in which the signals are simply multiplied.
In order to do this in stereo, I think I'd need to use the AU side-chain input, since there is no other way to get a second audio input to the plug-in.