Let's paint the scenario :
You've already and succesfully blended 2 tracks together (e.g. Bass DI at 0dB + Bass MIC at -4dB)
everything seems to be perfect so you go further in your mix...
Then you want to change the balance between these 2 bass tracks, here appears the trap ! each time you increase a fader, you must decrease the other one in the same range, because each modification on a fader changes the global volume of these 2 tracks, changing by the way our perception, because louder is better and vice versa...
What could be very powerful ?
The possibility to select the 2 tracks, then, with a simple shortcut, when you change the volume of a track (e.g. +3dB) the other goes in opposite direction (-3dB), like the "Link + Inverse Pan" works for pans.
You select two track (or more, why not), push the shortcut, make your balance modification, that's it...
Now, because of the difference in the two audio material, if the global volume seems to be louder or lighter, making a group solves the problem !
I mean, if you even already have your tracks fader grouped, the shortcut should continue working, as the "ctrl" do when you want to adjust only one fader in a group, but this time with "balance compensation" (ctrl + "Idontknowwhat")
It could also be possible to toggle on/off a "balance compensation" setting for two or more tracks (e.g. for 3 or more kick or snare tracks, or guitars with 2 different mic/cabinets...) with a little indicator on those tracks, or, just add the possibility on group attributes (as a check box "compensate tracks on volume change")
Of course, with 3 tracks, a compensation law would be necessary, if you increase a track by +3dB, others should not decrease by +3dB each, obviously.
It could also be very usefull when blending a track with its parallel compressed twin, where keeping the global volume is critical to make the tuning accurate, especially in context of global listening of a song