Good or bad idea?
Do you want to reduce file size? Good idea.
Do you want to preserve quality? Bad idea.
I should have correctly typed "signal-to-noise ratio" instead of "noise-to-gain ratio" (holdover from my college radio days, sorry).
Without devolving into horribly detailed math, you can think of each audio track as a speaker.
Each speaker contains signal (desirable audio) and noise (undesirable audio).
Let's assume a low signal-to-noise ratio, by valuing signal as "3" and noise as "2".
With one speaker, we get three parts signal for every two parts noise (3:2), for a total volume of "5".
With two speakers, we *multiply* the values, getting a signal-to-noise ratio of 9:4, more than twice as much desirable audio as undesirable audio, and a total volume of "13".
Most microphone/recording systems used at home have a fairly low signal-to-noise ratio. You can find this rating on your packaging in most cases. A typical rating is 70dB, or a 94dB:24dB ratio. A good professional microphone will have a rating between 200dB-400dB, or even better.
But this is just the math. Probably you should rely on the difference detectable by your ear...
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