You are still assuming there is sufficient slack in the system.
The system is DESIGNED to have slack, LP. That's the whole reason why "cashiers" are even a thing (they're not REALLY, but more on that later): so that people don't HAVE to drop whatever they're doing and process a transaction every time a customer buys something. A business run by a single person who manages the entire store completely by himself (say, 7/11 or a gas station food mart) has no slack whatsoever: he has X number of tasks to perform and Y number of customers to ring up. The customer's are a constantly shifting variable, so if he gets alot of customers in the day it takes him longer to do all the OTHER things he has to do other than run the cash register. If, on the other hand, he hires a cashier to work an eight hour shift, then for those eight hours he is free to focus on all other tasks involved in running his store. He MIGHT, if things get to busy, open up a second register until the rush slows down, but the difference between "normal operations" and "boom period" is called "slack" and it is EXACTLY what is accomplished by hiring a cashier in the first place.
The fact that they have two cashiers shows that sometimes the load exceeded what one cashier can handle.
No, it shows that their manager is a fucking idiot who doesn't understand how to delegate responsibilities. It's simple: if the load DOESN'T exceed what one cashier can handle, then you have one cashier go do something else.
Do they have more than one person filling these roles?
Holy shit, you've never worked retail before in your life, have you? Consider yourself blessed --
truly blessed -- that you can even ASK this sort of question!
Let me put it like this: the term "cashier" has actually become a colloquialism, because very few businesses actually have anyone on their staff that fits this role. It is, if anything, a sub-category among five or six other duties any given employee might have. When I worked at Walgreens ten years ago, for example, all the line employees were called "Service Clerks." Well ALL had a login at the register, we were ALL responsible for facing and organizing aisles, we were ALL responsible for cleaning and closing, and we were ALL responsible for overstock, loading and unloading inventory, cleaning the bathrooms and cleaning the offices. Hell, the fucking managers managed to get us to do half of THEIR work too when we didn't look busy enough. I wound up having to rotate stock in the refrigerator section to put the milk/eggs/cheese with the nearest expiration date at the front of the display. A fifteen minute task that wound up taking 40 minutes because I kept getting interrupted when somebody called IC3 (which means "more than three people in the line, so drop whatever you're doing and open the second register"). If there are no customers in the store at all, NOBODY is at the register, everyone's facing, cleaning, rotating stock or helping with inventory.
The cashiers at your local grocery store have at least three other jobs besides running that cash register. When volume gets low enough, you'll see one or two of them close their lane and disappear. They're not going off to take a nap, they're actually going off and running the entire rest of the store.
If so it's already shown that the load exceeds what one person can do.
The load ALWAYS exceeds what one person can do. The thing is, different loads come into play at different times. If it takes 5 people to clean the store at closing time, 5 people to face the aisles all day, 5 people to keep the stock rotated, 5 people to run the cash registers, and 5 people to unload the truck on tote day every thursday, how many employees do you hire?
Answer: 5. For the simple reason that none of those tasks have to be done at the SAME TIME. You get the same five employees to do all of them, and then you pay them a shit wage to do it all and leave them just on the ragged edge of being overworked because if they had any place better to go, they wouldn't be working fucking retail.
Now it's not only an infinite pool of profit but an infinite ability to add duties to your workers.
It's not infinite for either of those things. But you can pile up a SHITLOAD of responsibilities on an employee you're sure isn't going to quit (this is, in fact, the MAIN source of workplace tension in America and one of the biggest sources of animosity between employees and managers). In service industries, given a choice between hiring more workers and making the existing workers do more -- without actually raising their pay for doing it -- managers INVARIABLY choose the latter. It's easier, cheaper and more efficient to fill an employee's time with additional tasks than it is to hire an extra person and leave the first employee with flex time.