Look into Purchased Structures.  You’ll need two purchase orders, one for each supplier.  You buy the top level finished item from Supplier B where they have need of the component from Supplier A.  You need the purchase order for Supplier A to fill an inventory location ahead of the demand for the finished component.  This is initially setup on the Product Structure using the Issue From Location and Backflush of Purchased Components flag.

The PO to supplier B works like a mini-shop order to include the component as connected Supplier Material (RMB on the PO Line) from Supplier A to be consumed by Backflush at the point the PO is received from Supplier B.  The total cost of the finished component is then made up of the cost for Part A + cost for Part or Labor for Part C that goes into making the finished component at Supplier B.

The problem really to try it out is you have to set one up complete to see everything in action which does take a bit of time.

I don’t think you can just create the Purch Ord Direct PO in the way that you asked.

Many thanks for the response Shawn. Yes Purchase structures & component customer orders work well when we know what’s occurring up front, we just quite often find ourselves having to adapt purchasing strategies on the fly often after the PO’s are already in place. IFS obviously works much better when there is a known clear path right from the start. 

Really appreciate the quick answer though, thank you