I'm a little late to the party here, but it sounds like you guys were trying to figure out how to seamlessly compare a print track to your working mix, and to do so by listening to your working mix via Input Monitoring on the print track (Mix Bus Output = Print Track Input). The desired outcome of this setup is that disabling Input Monitoring switches playback to your previously printed track, re-enabling Input Monitoring switches playback to your working mix, and that these two audio sources do not play simultaneously. This is something I've seen people do in Pro Tools, and I'm hoping to find a way of achieving the same result using Logic.
When attempting to use this kind of routing in Logic, both the active region on my print track *and* my working mix playback simultaneously when Input Monitoring is enabled on my print track. Of course, I can simply mute the region on my print track to solve this, but for the sake of A/B'ing, it'd be nice to be able to switch back and forth between these two sources without this extra step. From what I've observed of such workflows in Pro Tools, it seems that enabling Input Monitoring automatically disables the output of any audio region on the track that you're monitoring through, which saves you that extra step, and allows for more efficient and focused A/B'ing.
Let me know if any of you have figured something out since this thread was last active!!
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QUICK UPDATE:
Right after replying to this thread I stumbled upon one that offers a comparable way of doing this - sharing it in case someone ends up here with the same question:
https://www.gearslutz.com/board/apple-logic-pro/1292020-how-you-setting-up-your-mix-print-busses.html
"You can Mute/Solo with one click - option-click the SOLO button of the channel you want to listen to - it will mute the others. Option-click the other one to switch. There's no delay when you solo this way..."