Using "Bluetooth device with earphones" and headphone jack simultaneously
I have tried doing the exact opposite thing (Forcing Audio to go through bluetooth instead of device speaker) so it definitifely is possible to play at least different audio through different streams (Media, Call, etc.) It set it inside of tasker to "Media" stream and it came out of my phone, so it might be possible to get it working. The app needs to support playing Audio through multiple streams. I don't know if there is an app for that that does split it up.

Sorry for posting it as a answer, my reputation isn't high enough yet to comment...