Why would I have been charged for 2 different anesthesiologists for a colonoscopy procedure?

Check with your doct. Are the charges for actually named physicians/anesthesiologists? Could one of them possibly be an anesthetist with a supervising anesthesiologist. It would be best to address this with the individuals who are billing you. In our practice, we typically do not use additional personnel to administer the sedation, hence no extra charges.
Good question! Unless two separate anesthesiologist participated in your care you should only have one professional charge. Often there is the professional fee for the physician and then there is a technical fee for the medication. You should review your bills and the charges with the medical providers to make sure no mistakes were made!