Preorder 1 product with 2 variables

i want to set up the products so that there is a limit, but the product has 2 variables, so the limit has to be split but still sync so that the combined total doesn't not exceed the limit.

i.e. size 38 sweater, but with the variable of adding a hat which increases the price of the order and works as a seperate product on Stoq. however I only have 50 units to sell. so if 50 is reached combined then they go out of stock at same time