Galileo's argument is actually quite slipshod and is based on the ambiguity of language and the narrowness of what-we-commonly-experience as a slice of all-that-really-is.

If Aristotle were right then the rocks would EITHER fall slower or faster together. There is no "knowing" if they are one object or two.

But Aristotle is wrong. Which means Galileo's thought experiment is irrelevant to reality and should be ignored. The picture is very simple: gravity pulls down, air resistance pushes up. Gravity is constant but air resistance is higher for faster moving objects. That's all you need.