You will need to go into the BIOS and change the ACC setting to 'Auto' I believe, and something else (sorry I forgot). Also, you will need to do a few other things, such as checking for stability using Prime 95, and making sure the two cores aren't faulty, as your two disabled cores may be defective.
Now-a-days there aren't many X2's or X3's that are perfectly working quads that have been disabled and are instead faulty quads. I've seen constant posts from people saying they've bought a new X2 or X3 and want to run it as an X3 or X4 and then had major problems and had to set everything back to normal to be able to complete a cycle of Prime95.
you are a tosser, the manafacturing process has been imporoved and more than 70% of X2 are perfectly working quads, infact a studt showed than of 16 X2s (done MSI) showed that 14 of them unlocked the quads, and the 2 that didn't well who cares