Bi-directional transaction link?

I need to associate a bill with a purchase order for reporting purposes.  I created a custom trans body field called Associated Bill #, type=list, list=transactions, filter-type=bill. This allows me to enter the bill number on the purchase order and now these two transactions are associated (this is a bill for freight - i am doing this because i want a profit report so i need to track all expenses).
The bummer is our staff will now have to enter the Bill and then edit the Purchase Order and fill out the Associating Bill # field.

Is there a way to create a bi-directional link so i can link to the purchase order from the bill (that way the link can be established when our staff enters the bill and they don't have to find the PO and edit it)?  Seems like a long shot but I thought I would ask.