From sends, or it would just show up in the outputs as:
-Crossfader 1:a
-Crossfader 1:b
and this way it could also be accessed in Direct Outs summing mode, so you could have a bunch of bass and drum stems from one song summed to Crossfader 1:a, and then a bunch of drums and bass track from another song summed to Crossfader 1:b.
I like your idea for having like a pan slider which leaves the fader for overall volume.
Having both these options would be amazing, and open up some really amazing creative possibilities that have instant tangibility, rather than having to automate this and lose creative flow. It’s the instant accessibility that births new creative potential.