Try this in the properties window for the combo box
rowsource table_name.field1, field2
controlsource table_name.field_same_as_value_returned (field1 or field2)
boundcolum to which ever of the two in rowsource will return the value (1 or 2)

Even easier is to run the builder (right click on the combo box, select builder). This will walk you throuh the steps. I find it best to but the field I want returned as the second field, and in builder, a double click on that field (on the third page) will hide that colum from the user.