Hi,
As you are telling it is centralized purchasing , if your two plants belongs to the same company code , i think you can use the concept Distributed contracts created by the CC and you can have plant based conditions and based upon the MRP requirement in both the plants you can try purchase .
Also i think even though you procured more quantity other than MRP suggested in plant 1 , you can verywell transfer the quantity . Once they are transferred system considers to be as stock available in plant 2 and acordigly place the MRP elemnts .
Strategy group 40 is planning with final assembly.The planning strategy represents the procedure used for planning a material and is (technically speaking) controlled by the MRP types.
In your case for mat A which will be in both plant 1 and 2 , Plant 1 should generate the requirement of plant 2 in MRP .
Regards,