Select from lookup only entries associated with previous lookup

Select from lookup only entries associated with previous lookup

I have a two lookup fields in my Projects form. The first is Customer, the second Customer Contact (Customer Contact is a subform of Customer). In the form definition I have this code:
Customer
        (
            type  =  picklist
            values  =  Customer.ID
            displayformat = [ Customer_Name ]
            allow new entries = true
            [
                displayname = ""
            ]
            sortorder  =  ascending
            searchable  =  true
            width  =  206px
        )

        Contact
        (
            displayname  =  "Customer Contact"
            type  =  picklist
            values  =  Contact.ID
            displayformat = [ First_Name + " " + Last_Name ]
            allow new entries = true
            [
                displayname = ""
            ]
            sortorder  =  ascending
            searchable  =  true
            width  =  206px
        )
This works fine for now with a small database. I would like to display First_Name Last_Name only for the Customer selected in the previous look up. I have experimented with filters in the field definition but cannot get any to work.
Help would be appreciated.