PR for component only

Hi,

I'm working on a scenario where there is one product (component/FG) at the same time. it is used as a component to make another FG (same product but a different material #. Will call it FG2). Both are planned using ROP. We may or may not want to carry stock in the FG2 but when there is demand for FG2 we need MRP to recognize this demand and generate a PR for the component only if required and not create another planned order/purch.req for FG2? Any suggestions would be welcome

Thanks,

SG