Services Procurement: Timecard entry in OTL combination of (project an purchase order) and purchase

We want to use Services Procurement (in combination with Oracle Time and Labor and Purchasing and HR and Projects)

Our contractors write labor in OTL.

This is the issue:

2 days in the week the contractor needs to do work for a specific project. In that case he needs to write labor on a project in combination with a purchase order

3 days in the week the contractor works for the department who hired him. In that case he needs to write labor on a purchase order (without a project)

From my understanding, a contractor can only write labor in OTL/Services procurement on a project in combination with a purchase order or he can write labor on a purchase order but he cannot combine this on 1 timecard. This is very inefficient!