See I thought she never put in the receipt for the second table, and pocketed all $4 from the second table. I had a friend that did this with cigarettes at a drive thru. He would pocket the money selling packs, then sell the box once it was finished, and pocketed the difference.
There is one issue with this explanation, though: Dee's lines "We're sticking it to those corporate bastards, aren't we? [...] Just turn a blind eye while I rob this place stupid" make it sound like it is the bar that is being cheated, not the customer. Is my explanation incorrect, or does Dee just not understand who the actual victim is?
This might work as a scam, but I'm not sure it's the scheme referred to in the show. Dennis' lines about how they "always just took the difference from [Dee's] purse" also implies she's stealing from the bar.
I think the scheme might work more like this:
Two tables have a similar order
Dee gives one table their bill and they pay in cash
She pockets the cash (and the tip)
She delivers the same check to the second table
The second table pays and she puts that amount in the till (and keeps the tip)
This scheme would not work if the till has inventory tracking, but given the gang's poor understanding of technology and finances, such as Mac's "Irish honor system" being considered, and every character drinking beer from the bar's inventory without paying for it it, they clearly have no inventory tracking system