Publically they do not have any interest, I agree.
However, they may ascribe to exactly the theory you put forth -- it was a two man job. If they say Helgoth is the killer, then #2 goes scot free. If they say Helgoth is one of two men, then killer number two knows they are onto him and goes way under.
By never saying Helgoth is/was a suspect, they keep #2 on his toes -- Do they suspect 2 people? Am I free and clear? Do the ramsey's know I exist?
IN this scenario - Karr would make a perfect #2 guy -- It would wash that he would say her killing was an accident etc.
Hopefully an angle they are exploring.
Karr has been to Boulder... Sandrock said today that Karr described to him many places in Boulder, including Karr describing what type of brick was used on the Daily Camera building.
But who really knows if Karr had been to Boulder before JonBenet showed up dead. I truly believe that Karr has been in Boulder at some point in his life, even if was just to drive by or walk by the Ramsey home so he could feel close to JonBenet even if it was after her death.
The DA probably knows that Karr has been in Boulder before, they just are not talking about it at this point. This would have been something they checked out right away when they started tracking Karr.