You can create both multipacks (4X6 & 24s - I can’t have both?), but Breww won’t inherently know that they’re packaged differently. It will just think that you have two products with exactly the same beer components - it won’t know that one is essentially a multipack of multipacks. However, being different products, you can add different stock items to each product, so you could add ‘flats’ specific stock items like the cardboard flat tray to one product, and not the other product.
Does this solve it for you?