This is a great topic. I actually found this while researching for an article I'm writing for "The Auditor" on this very thing... this topic is close to my heart, as back in 2001 a few of us wrote a book [Integrating Quality, Environmental, and Health and Safety Systems] on this topic, and what has been posted here resonates with what we found back then.
Namely, auditors tend to be more thorough in their "primary knowledge" standard - it simply comes easier to them to cover that standard completely. I also agree that multi-disciplinary auditing can be taught, and auditors can "step up to the plate" and do a great job on multiple standards. Since we included a comprehensive checklist in our book, I would also agree with the statement that a checklist is a great tool!
However, don't for one minute think that if someone only audits one standard, that no audit bias exists. Remember the story about the auditor (background in metrology) who comes into a lab and discovers an employee on the ground, from apparent electrical shock? He first steps
over the employee to read the calibration tag on the equipment... then kneels to offer assistance.
Mary McD