I'd like to be able to have audio from 2 different applications go to 2 different audio outputs (say speakers from line out, and bluetooth audio headset). Is this possible on OSX 10.11.6 through standard system prefs or an external audio manager or something? Cheers.
As @IconDaemon says, your two potentials are Rogue Amoeba & Soundflower, paid or free.
Personally I don't trust Soundflower any more, it started to get very crashy around Mavericks & I've not really touched it since.
It was made by Cycling74, then given to Rogue Amoeba to host, but they've long abandoned it so its maintenance has been from whoever posts the most recent re-write.
Rogue Amoeba have 2 apps, one that records, Audio Hijack $49, one that just routes, Loopback $99. I've not really played with Loopback but Audio Hijack already can do most routings.
I made some pretty pictures of possible routings using Audio Hijack on this answer - https://apple.stackexchange.com/a/203009/85275
A third choice is Plogue Bidule - which I just found overkill & never really spent the time to learn it