How do i set up variants to subtract from stock correctly?

Hi,

We have a system set up so that our vodka has two variants: 25ml and 50ml.

Obviously if 50ml is used then it's technically two of the 25ml, but still only subtracts 1 from the stock, is there any way to make the 50ml variant a composite of two 25ml? 

we were able to do this with separate items completely but having a wide range of spirits makes everything very disorganised.

Any help is much appreciated