Did you try to add the BETFCY criteria in the assignment rules?

Then set APPPOH = Yes, optional.

I believe if you set no line for BETFCY = Yes in the rules value, you will have intersite purchase order signed automatically after creation.

Thanks Julien Patureau for this tip.  In my case the PO signatures are buyer and value based so it was important to have a catch-all so that all orders are captured in the workflow.  Without this, should a user have access to purchase orders and they are not in the assignment rules, the order could still be actioned.  

In addition to setting APPPOH to Yes, optional, and including BETFCY in the assignment rule:

• Add BETFCY as a condition to the normal workflow and check for non-intersite (BETFCY=1)
• In the assignment rule add a default approver so that anyone not defined in the list of buyers will get assigned to the detault user.
• Create a second workflow rule where the condition includes BETFCY=2 (inter-site) (a copy of the first one)
• Copy the assignment rule
• In the assignment values, make sure not to have a default approver and add one line where inter-site is No.