I suspect the wiring isn't your problem. If the Cable boxes are the same model they use the same IR codes so it is more likely the 500 is simply operating both boxes when it transmits. Could you move the Slingbox and it's associated cable box well away from the other one ? A different room ?
What type of cable box is it ? Some manufactureres (Tivo, Dish, DirecTV) allow you to change remote addresses so you can run multiple boxes in a single location.
Thank you for contacting us here on the answer forum. I am sorry for this problem and would be more than happy to help. It sounds like both boxes are on IR and are running the same IR codes. The easiest fix to this issue was to change your room mates cable box over to RF setting. This would let your roommate work his cable box and would allow the Slingbox to change with no interference.
Hope this helps,
Sling Moderation Team