Stock management with multiple attributes

[ThreeThree]

Hi, my client sells items of clothing that until now have been one size and one colour. They would now like to sell combinations of 2 sizes (tops and bottoms) and a colour, thus multiplying the number of combinations in the product page.

However they're a little stumped about how to best manage the physical quantities. If there are 4 "top" sizes (1, 2, 3 and 4) and 4 "bottom" sizes (1, 2, 3 and 4), will it be necessary to distribute the physical stock amongst the possible combinations? For example, if they have 8 x size 4 tops, would it be necessary to assign 2 of them to each of the bottom sizes? Ideally they should be able to input the actual number of Size 1, 2, 3, 4 tops and Size 1, 2, 3, 4 bottoms only.

Does anyone have any advice on this?