disclaimer: i don't have a bluetooth in car stereo hookup. just going to make some comments based on the fact that i have two different bluetooth headsets paired with my one x: the jawbone era and the phiaton ps20. (the phiaton enables call answering and not much else, but i got it mostly for listening to music. the jawbone works with enhanced caller id and sms and will announce callers and read sms texts, which is for when i'm running errands, etc., with the phone in its pouch.)
in order to get the ps20 to pair, i had to first unpair the jawbone. i did that and then did a soft reset of the phone (did not power down, just turned the phone off and back on). then i re-paired the jawbone.
i notice that when i turn on bluetooth, the phone shows that it is searching for both headsets if i have them both on and nearby. the jawbone has some additional instructions relating to pairing that i found on the website but haven't paid much attention to because my main interest has been trying to get voice dialing via bluetooth to work. (big fail there so far.)
given the apparent finicky nature of bluetooth (imo) at this point, pairing order could be an issue if you use more than one bluetooth connection. just a thought.