Purchase Items in mixed packs and stock as single units

What we are trying to achieve is the ability to stock two inventory items as two separate items but then purchase them as a pack.  This is as we can get a better purchase price by purchasing from the supplier as a pack but we may receive the items on two deliveries and issue the items separately.  At the moment we hold the item in stock as one item and do not receipt the item until all parts of the pack have been received, but in the meantime we may receive the invoice from the supplier and have issued the stock from stores resulting in imbalances in our stock position and holds being placed on invoices.