I'd like to add that although that rule is the best of all worlds, many people with 2 cats use only one litter-box. So long as you scoop the poop out each day and change the litter completely each week, you may be okay....(key word here is may)
Given that the cat pooped on the floor though.....for you, this option seems risky and you don't want the cat to get used to pooping on the floor.
If this were my situation, no question, I'd use 2 boxes.
Another general rule for litterboxes is scoop out the poop DAILY and change all the litter WEEKLY.
Hope that helps