GC2T013 Copy & Paste from description:
1. All requirements must be met on or after the date this cache was published. You cannot count any 20/20 days you had prior to 04/13/2011.
2. You must do enough caches in one calendar day to add up to 20 or more stars on the difficulty and 20 or more stars on the terrain.
Example: You could do twenty 1/1 caches = 20/20 or you could do one 5/4 cache, one 4/5 cache, two 2.5/2.5 caches, two 2/2 caches, and two 1/1 caches = 20/20. Any combination will do just as long as it adds up to 20/20 or over.
3. Since all requirements must be met before looking for the 20/20 Challenge Cache, the stars on the 20/20 Challenge Cache can not be used to meet the requirements.
4. You do not have to do the 20/20 Challenge Cache in the same day as you meet the requirements. Just put the date you met the requirements in your log for this cache.
Yes, a checker can made, but the challenge conditions do not lend themselves to a simple tag, so I believe a new script would be required. This is likely to be low on most script-writers' priorities since, given the current guidelines, the script would have limited usefulness.