Most effects affect all channels in the same way, with the same parameters. It is not even possible (without splitting into different files, or pipes, and reassembling) to change the volume of individual channels. The -e option of the gain effect only covers certain cases.
I imagine a "separately" (pseudo-?)effect that accepts sets of channels and effects sub-chains as parameters and runs the sub-chains on the relevant channels, like so:
sox quadrophonic.wav out.wav separately 1,3-4 "vol 0.82" 2 "vol 0.47"
which should change the volume for all channels, but by a different amount for the second; or even
sox stereo.wav out.wav separately 1 "vol -3 dB" 2 "dcshift -0.2 highpass 8000"
Of course, the sub-chains should not change the audio length, sample rate etc. (or padding rules would be needed).