The two caps appear, for example, in Dr. Hunt's postmortem report.
One convenient way to explore the postmortem report is Tim Wilkinson's transcript, David Kelly: pathologist's report to the coroner - text version.
One cap is mentioned in Dr. Hunt's postmortem report as being found beside the left shoulder:
Lying adjacent to the left shoulder/upper arm was a 'Barbour' cap with the lining side uppermost. There was blood over the lining and also the peak.
Another cap is found in the Barbour jacket:
NCH/17/5 Flat cap from game pocket
It's possible that David Kelly simply forgot that he had a cap in the game pocket of his Barbour jacket and took a second cap with him on his walk on 17th July 2003.
But it's at least equally likely that the Barbour cap found beside the body may be a mistake by a "scene setter" who is trying to make the scene of a murder look as if it's the scene of a suicide.
Typical of the Hutton Inquiry. no effort is made in oral questioning by counsel to establish whether or not the cap beside the left shoulder does or does not belong to David Kelly.
In the absence of any questioning by counsel, one must assume that the counsel to the Hutton Inquiry considered it normal for someone to carry two caps on a walk. Counsel certainly didn't seem to notice anything unusual.