There is no linear relationship to with what I think you are trying to do, I mean, summing or extracting them, with gain scales and that's it. You can't have 3 independent tracks into 2 signals and recover them, if that where true we would only need 2 track recorders and then we can add a 3rd, and we still have 2, then we add a 4th, and still have 2 signals, and so on. There are other ways to do this, multiplexing is one, but you have to sacrifice at least half your bandwidth to fit 2 signals in one channel without perfect separation, as stereo FM for example, you multiplex M and S signals, and then you add as much S as you can split, and you have your stereo signal. Probably not good for your application since you would more likely to have the a single track for the click and another for the stereo, and you want the best for your stereo, with maximum separation from the click.
You could sacrifice a little bit less of bandwidth add a HF pulse, let's say a couple of 20kHz periods with a window so it doesn't introduce noise in a much lower band, then have a filter, at 16kHz so your signal from 16kHz and lower gets out and then you have a detector for the 20kHz which triggers a click from another place, a simple analog oscillator to do so, putting a pulse out, some eq/filtering and you have the click you want in an audible band, but your signal would be cut over 16kHz, if you do so in the S band you don't get stereo over 16kHz, which is probably fine for a live performance, you use stereo down there, and the M signal could have all high freq you want. Then for decoding you need a filter on the S, up to 16k you have your S, add and subtract from M to get L/R and over 16k at your S you use as a trigger for the click.
JS