Your only error: "I thought you can use either option to display the second field."
If you use the drop down option, then you will see the ID number instead of the second value after exiting the field. If you really want the drop down option, you have to play tricks with your layout to get the effect you want.
Here's one example:
Place your ID field on your layout, format it as a drop down and select the "Arrow" option. Now re-size this field until only the arrow is visible. Place the related 2nd value field next to it so that it appears to be one field. Format this second field as a button or give it a script trigger script that uses Go To field to move the cursor into ID field any time the user enters this field.