Considering most human beings have two supporting points, called feet, and any movment you take after your putt is considered following through, this clearly means that you have to have both feet in contact with the ground behind your marker before you can walk past it.
While I agree with your logic and the fact that placing both feet on the ground is probably the best way to determine it, I simply don't think that is what the rules say. You left off the part of the rule that defines what a follow through is. The rule clearly says: "The player must demonstrate full control of balance before advancing toward the hole." It is certainly possible to demonstrate a full control of balance on one foot. Since the rules don't specifically say that you have to put two feet (which, btw, are different than supporting points and using the term feet would be inconsistent with the rest of the rule book, as would specifying a certain number of supporting points) on the ground, I think you'll have a hard time stroking anyone on the basis that they didn't have two feet on the ground and proving your case on the course with a rulebook.