Ok so I've been playing my six string. and for about two weeks i've noticed a buzz on the 6 and 8th fret on the E, A , D , and G strings. so I decided I would fix it today because I'm sick. I've done plenty of setups on my basses and I normally get them right. But I straightned the neck and the shimmed it. But then it kept buzzing. So I looked at the neck from the bridge of the bass and what do you know there was a slight bow coming inward at these frets. So I thought I straightned it but I guess I didn't. But I don't know how to get the bow out of the middle of the neck. I'm afraid if I turn the dual tross rod left it will backbow around the twelfth fret and if I turn it right I think it would make it worse. So I need some advice on what do do before I start doing it. Normally when I straighten necks there isn't that bow in the smack middle of it. So any advice would be appreciated.Thanks,

Ive figured out what to do but I have run into a problem.
I turned the dual truss rod to the right to tighten and the bow has come out and the neck is straight all the way till the twelfth fret. but then it bows down a bit so I need to tighten it more. But my left truss rod out of the two has gone slack. But the right one is real tight. I think that the right one is holding all the tension. I can loosen the left one a bit and feel a little tension. So I don't know what to do should I loosen the right one so the tension will equal out and start back at square one. any advice is much appreciated.