Custom Tab for Table With Composite Primary Key

Hi all! 

 

I'm trying to create a custom tab that can update a local table that has a composite primary key - that is, the primary key consists of two different columns.

 

The Id_Key column in the custom tab is supposed to represent the primary key of the updateable table. But Id_Key is only one column, so I'm not sure how to model a composite primary key. Should I use a second Id_Key column or something?