Hi Hung,
Enter Material B in BOM of Material A with the quantity of Material B same as the base quantity.
BOM Header : Material A Base Qty 1 PC
BOM Iteam : Material B 1PC
When PIR is being entered for Material A, lets say 1000 PCs , MRP will generate requirement for Material B for 1000Pcs. The plan order generated can be converted to PR & PO once material B's requirement is finalized.
Thanks
karthik