Attach an 'ID' column to the main attribute, the one that will be dictating the other supplies. You can do this by going to the attribute, adding a column called 'allocation_id' and giving each row a number, starting at 1.
Then, in your product, you can add a new supply formula, choose the location and the supply to be allocated. Here's a formula that will achieve that:
allocation_id == 1 ?
This will first determine if the customer chose the material with an allocation ID of 1, which would correspond to the first item in the drop down menu.
Then, if it's true, you can add a formula to allocate the selected material. You will need to add a new formula, changing the allocation ID each time, for each secondary material.