when creating a value list, filemaker gives the option of showing to field when selecting a value from that list, and that's just to make it easier for user , cause :
for example if u got a value list that shows Id_projects, it would be much easier if the value list shows both "Project-ID" and Project-Name", but once a project is selected only whaterever value is chosen in "use values from first field" (in the value list option ) is the real value and "also display value from second field" is just a display option to make it easier for users.
so make sure u choose the right field as a "use values from first field" cuz that's the real data that u would be dealin with.
To show both values, you need to add a field to your layout to display the other value. Just use the field tool to add that field from your related table to your layout. Since you usually don't want the user to be able to edit the data in this field, you can use Behavior on the Inspector's Data tab to deny access to this field when in browse mode.