The customer doesn't need to explain to you why they want both subs marked. They just want both subs marked. The subs could be for someone else. It isn't your business. So you thought just one needed to be marked. They asked for both. Do it. There is no need to argue with them unless they are trying to get something free for nothing. I run a restaurant.... I will be firing that employee.
That was my thought, that perhaps the subs were going to 2 different places. But who cares. What if it's THREE different subs? Gonna be marking at least 2 then, right? Employee is 100 % wrong and new store policy is every sub gets marked.