Trongsittirak,
If you want your STOs to have multiple source plants, this is easy to do. You can configure many source plants for a given destination plant in STO configuration. Each of these source plants must be assigned to a unique vendor. Thereafter, you can manually create a Stock Transport order for any of these plants.
Standard ECC MRP, however, will generally only determine one source of supply for any procurement proposal, (in this case; one plant/one vendor) except with Quota arrangements.
Best Regards,
DB49