Hi Swathi,
1. In Plant 1000, you can use strategy group 40 and assign the sto key in material master as F-40(supplying plant as 1200)
2. In Plant 1200, you need to create the Customized strategy group by combining the planning strategies 40 and 20.
3. If you want to create the sales order specific to customer, then during sales order creation, change the procurement type from KSV to KE in procurement tab.
4. Run the MRP, will give you desired results.
Please let us know your observation and if for additional information required
Best regards