Fixed Input Ratios in Optimisation

[dgibson]

I am running a large scale optimisation to determine most desirable allocations of products to customers from a range of possible supply sites while adhering to shelf life requirements. For a subset of orders from customers, and for reasons beyond the scope of this forum, combinations of some products must be suppled in fixed ratios to satisfy the customer's demand.

In other words, if a customer orders a product, A, it can be supplied from a combination of products X, Y, Z, where products X and Y must be in a fixed proportion to each other, but product Z can form anything from 0% to 100% of the order. For example, the following 3 outcomes are all acceptable:

1. Z = 20%, X = 40%, Y = 40%

2. Z = 70%, X = 15%, Y = 15%

3. Z = 100%, X = 0%, Y =0%

Because of the size of the problem I'm using a (SLAM) matrix input method. By way of experimentation I have successfully implemented a constraint to limit both X and Y to a maximum %. My difficulty lies in applying a constraint to enforce X and Y to be supplied in equal proportion. Unfortunately, limiting both X and Y to a maximum of 50% wont work because of the presence of product Z. 

Intuitively the required constraint is obvious I.e. X/Y=1 but I'm unsure how to apply it given the matrix input method. Can anyone suggest an approach, or point me towards where I might find an answer?

Any tips are appreciated.