Multiple Purchasing organisations

Dear SRM Gurus,

We are on SRM 5.0

Our backend R3 structure is as below:

Plant 1----Company Code 1 -

Purchasing org 1

Plant2 -

Company Code 2 --- Purchasing org 2

We have created both Purchasing organisations in SRM and assigned the same buyer with different purchasing groups to each of the Purchasing orgs.

Based on the location of the shopper, we want the purchasing organisation to default during the PO creation.

Firstly how do we assign the same buyer user to both purch orgs?

Secondly how on PO creation do we get the PO to default to the correct PO org and company code?