Even if you get this to work, I think you'll have problems trying to use bluetooth encoded audio and the direct output from your motherboard at the same time for two reasons:
1. There may be a delay from the audio fed into the bluetooth transmitter to the time it plays in the receiving speakers because it takes a finite amount of time to encode the audio to bluetooth. If the delay is perceivable, it could be a very unpleasant effect.
2. Bluetooth encoding is so bad for phone conversations that I won't use it, and I often ask my friends with bluetooth earsets to hang up and call me when they get to a real handset. It would be even worse if you expect actual musicality from the sound you get through the system. It wouldn't matter if you're just transmitting material that's already encoded to a low bit rate in a phone because it's alreay FUBAR.
Sorry to be a wet blanket on your idea, but as a musician and engineer, I wouldn't waste time, let alone money, trying to use bluetooth for music if I cared about the sound... and I do.