Third Party processing

I have created a sale order for a third party item for 50 units. It creates a Pur req for 50 units. I convert the req into10 units in the PO. Then I try to convert the remaining 40 units into a PO and it soes not let me do that. I believe creating multiple POs for from a Third party Pur Req is not possible. In our scenario we will create multiple POs for different vendors ( example 10 , 20 and 30 units) for the Third party item for which a sale order for 50 units has been created.

Is there a way I can create multiple POs for a third party item

thanks a lot