Hi Doug
These are two separate questions.
You cannot outright combine two PO's into one, but you can merge them together as if they were one when receiving. In other words, you can receive against multiple POs in one step.
A PO by definition is an approved requisition. A requisition is an unapproved PO. To do what you want, create requisitions, not POs, and then turn them into POs thru the approval process.
Steve