Luis,
Your requirements are somewhat vague.
If you are only talking about dependent requirements, then probably the easiest solution is make one of the two materials to be a phantom, and the other to be a component of the Phantom. This assumes that you do not intend to build one of the two materials.
If you are talking about third party demand, you can use Material Determination. In this case, customer service enters the order for one material, and the system automatically keeps track of the original material ('Material Entered') but the order is actually placed for the substituted material.
Material Determination - Basic Functions and Master Data in SD Processing (SD-BF) - SAP Library
There is another method within MM called supersession.
Any of these may or may not meet your requirement, based on the EXACT need.
Best Regards,
DB49