In SAP before version 9.0 you can use 2 people at the same time. In version 9.0 or superior you can't do it.
If you want SAP shows you pop up windows to sign manager password, you have to:
1st. Assign a professional license to user SAP03, but in authorizations window you won´t give him authorization for create PO.
2nd. You have to give the manager password to person that use user SAP03 (or you write the manager password in the popup windows when this appear).
A professional license is necesary to create a PO. And if you don´t give authorization for create PO the popup windows will shown.
Test this on your SAP please, and comment us the result of this.
Regards.