Both apps need to have audio communication coded in order to work.
Our new replacement app (called iReal b) has the option of background audio on or off. Also we have it running with iRig/Amplitube's background audio so guitarists (etc.) can play through Amplitube but interact with iReal b, viewing the charts as it plays.
Also the device needs to have enough processor power to be able to do both. Many apps use nearly all of the processor resources already.
Thanks for your post. We will be looking into this in the future.