You don't need to give a company details about your other interview. Some places have boilerplate out there which says "please do not apply for any other jobs while you're applying to us" but I think most everyone realizes that this is not a realistic thing to ask prospective employees to do.
If you happen to receive the job offer before the second interview, there's also no rule that states that you have to respond to said job offer immediately, at least not usually. Tell the first potential employer that you need to weigh your options. There's no need to let them know that you're looking into another offer; in this case, it could be perceived as a negative in that it makes it look as though you are their second choice.
I realize that you don't really care either way between the two jobs, but I think you do need to ask yourself a question: if you get them both, which one will you turn down? There may be small differences in pay between the two places but by and large retail places pay about the same amount (which is somewhere between "minimum wage" and "dirt") so outside of a 'vibe' you might get from the interview itself I'm honestly unsure why you wouldn't just choose one of them as your first choice. This certainly makes it easier for you if the first interview turns out to be the job you preferred.
Otherwise, if they do want you to respond immediately I would go to the old saw that a bird in the hand is worth two in the bush. In other words, don't decline an open offer in favor of another offer which may or may not come down the line. Even retail declines to hire folks sometimes - my memory is that places can be particularly picky outside of Christmas - and if you turn down the first job before being extended the second one, you might be left out in the cold for a while.