Neither register is the master
Two point-of-sale systems, no website in the middle, and no natural owner of the number.
This is the pair where both sides are counters. There is no storefront acting as the obvious source of truth, so the question is not which platform to trust but where the count should live. QuickSync holds it between them and corrects each side from the other, which means a sale on the Square register lowers what Clover will let you sell, and a Clover sale does the same to Square.
Shops end up here for ordinary reasons. A second site opened with different hardware, a business bought with its till already in place, a market stall running Square while the shop runs Clover. In each case the stock is one pool and the two systems have no idea the other exists. Left alone they part company within a weekend, and you find out with a customer waiting.
Both sides announce their changes to us as they happen, so nothing here is on a timer. That matters most when the same item can sell in two rooms in the same minute, which is precisely the situation two tills create. A count that moves as each sale is rung through is the difference between telling the second customer no and selling them something you do not have.