Using "Bluetooth device with earphones" and headphone jack simultaneously I have Sony Xperia SL and have a bluetooth headset (SBH20). I can play music from SBH20 when I connect it but then if I connect another earpohone to my phone's 3.5 mm jack, the audio from bluetooth device gets disabled. Only one thing works at a time, either phone's 3.5 jack or bluetooth device. Is it possible if both works simultaneously so that output can come from 2 earphones at the same time.
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...