I want to show a list of two different fields as one column in a dropdown list box.

Example:
Recno, Field1, Field2
1 AB ZA
2 AC ZB
3 AD ZC

List Box needs to show the two fields as one column:
AB
AC
AD
ZA
ZB
ZC

User selects ZA or AB and goes to record #1.

How can this be accomplished?