This is theoretical. Would I tell them politely to stop arguing or would I simply apologise to the customers around me? Reason I'm asking is the former could possibly cause conflict. This is for a job application, have no idea about this stuff! Thank you!

Politely ask them to leave. During the interview ask how often you'll be expected to be a bouncer. If you're unhappy with their answer, pass on the job.

After Comment:

Not all customers are worth having. Two that argue, likely won't buy anything, and are driving away most all other customers are worth passing on.

First ask them if they need help with any thing, they could be arguing over what to buy. If they say they don't need help and if they're really disrupting business then you could politely ask them to take their argument out side, or to some where that they're less audible and or let your manager know about it.

You don't want to get your self involved in an argument with a customer. If things escalate you don't want to get in a fight or you could get fired.

Offer to help, and call security.

You should ask what the problem is and suggest that they move it our doors.

Freedom of speech is best speech.