Just because cat #1 was trying to cover the evidence does not mean that cat #2 isn't the culprit. I too find poops on the floor at times (sometimes they ARE hang-ons that drop, and sometimes they are accidents) but either way, some cats will just walk away and other cats will then try to cover someone else's mess. So never assume unless you actually SEE the deed being done.
The one time I tend to find the "accident" poops is when the level of the scoopable is too shallow. Something else to keep in mind as a possibility.