Value list based on multiple criteria

I am an FMP newbie trying to create a database to track inventory and work orders for a group of field techs. I have created two files, inventory and workorder. The inventory file has: "serial number", "stock date", "pull date", "tech name", and "useage date" fields. The workorder file has "tech name", "serial number" and many other fields. I enter equipment into the inventory db once a week, entering "serial number" and "stock date". The techs come into the shop and update the record, entering "tech name" and "pull date" when they stock their trucks. Once they actually install the equipment for a customer, they need to enter workorder info. I want the "serial number" field only to display records for the "tech name" who is updating the record and where "useage date" in the inventory db is empty. (Equipment not installed yet!) I have the first part working with a relationship. (inventory:tech name = workorder:tech name) When I select "tech name" from a drop down list on workorder, the "serial number" field only shows equipment from that techs current stock. It's the "useage date" part I'm having trouble with. Hope the explanation is OK.