If you select the field 1 in the combo box A and want to automatically display the contents of field 2 in the text box B,

Attribute in text box b->; Data-> Write to data source: =DLookUp ("field 2 name", "table name", "field 1 name ="&; [combo box name])

If the field 1 is text, =DLookUp ("field 2 name", "table name", "field 1 name ='"&; [Name of combo box A]&"'")