The way I would do this (and I have) is to modify the query behind the combo box. If the two fields are LastName and FirstName, for example, modify the query like this:
Add a column to the query called "Name" and put this as the source:
Name:([LastName] & ", " & [FirstName])
Then hide those two columns by unchecking show column.
Then modify the combo box to 2 fields (PK and Name) and change the display to something like 0";2"