I don't believe the CO 500 is an enduro. It is an invitation-only dual sport ride last time I checked. There may be a different CO DS ride that you could enter that is a multi-day event.
Check out the AMA web site for a listing of the two day ISDE qualifiers.
www.ama-cycle.org Our club is putting one on this year. The Ohio one is definately worth the trip, if the weather cooperates. Most of these have "clubman" classes so that you don't have to live up to the qualifier rules (inspection and marking, Park Ferme, etc.) that the contenders do but you can still ride and compete in the whole event.
If you are up for dual sport instead of enduro, there is the Six Days of Michigan ride, the two day Buffalo 500 (kms) in Indiana, and the Terlingua Dual Sport 2 day ride in Texas. If you go early enough to the last one you can easily ride on Friday as well - lots of areas to ride in around there if you like gravel roads and high desert mountains. Not sure, but I think most of the Suzuki National Dual Sport Series events are two days.