this might be ancient history at this point cause its so old but.... in logic, select multiple tracks in the mix. hold down option shift, click on the input of the first track and set to whatever input, bus, or source you want. the remainder of the tracks should go up from there. if you then add those Aux tracks to the arrange page (if you don't already have them) and add a region to each track using the pencil tool, you can select export tracks to audio. If you have hidden the rest of the tracks of the session, the only ones that will bounce will be the aux tracks that have the regions drawn on them. this is a pain to setup, but I have it in my template. this negates the need to have a second set of audio tracks to bounce into for BFD, which I also use. I might have missed a step cause this is all from memory, but hope it helps.