Two columns is all you can do in a filemaker value list--that's one area where MS Access offers many more options for a combo box than filemaker.
You can define a calculation field, cFull name as First Name & " " & Last Name and use it as column 2 in your value list. If you hide column 1 and use the pop-up menu option, you'll get a value list where the user selects a person's name but the field stores the ID code from the hidden column 1. You can also use a drop down list and place the full name field from the related table next to it to display the selected name. Be aware that if you hide the first column, but have two people with exactly the same first and last name, the duplicate value will not appear in the value list and you will be unable to select it.
If you want to pursue a more sophisticated, scripted "name search" method for finding a person's ID number, see this tutorial:
Thanks this sounds perfect!