I think that explains my taste in "focus" music. Most of the "focus" playlists are ambient or classical. The ambient stuff is just there; it doesn't really engage my brain. It's like traffic noise. Classical is... I don't know, too structural? I find it distracting. My brain switches on when I'm listening to stuff that's challenging - dissonant, rhythmically odd, fast paced, changing tempos. But I've thought the same thing, that kind of music engages some part of my brain so the part that's more linear-thinking can get on with business uninterrupted.
So my "focus" playlist is post-bop Jazz with things like Chick Corea (especially Three Quartets), Steps Ahead, Keith Jarrett's Standards Trio stuff, early Wynton Marsalis. Some Coltrane, but not so much his meditative stuff - things like Giant Steps make my brain engage.
I can't multitask like that though - I can't have two auditory sources. When I'm talking to people, I have trouble picking out voices when surrounded by a crowd, or with several people talking at once. It's like I'm trying to make sense of all the background noise and can't pick up what's being said directly to me.