Sorry I misunderstood but now I get it!
This should be easy to achieve using code.
You could set the Rowsource of your 2nd combo to contain all the required fields and then on the AfterUpdate of the first combo set the ColumnWidths to show the appropriate field. Below is an example of waht I mean:
Code:
Private Sub FieldCBO_AfterUpdate()
Me!ValueCBO = Null
Select Case Me!FieldCBO
Case 1
Me!ValueCBO.ColumnWidths = "6 cm;0;0"
Case 2
Me!ValueCBO.ColumnWidths = "0;6 cm;0"
Case 3
Me!ValueCBO.ColumnWidths = "0;0;6 cm"
End Select
Me!ValueCBO.Requery
End Sub
This is just an example but hopefully you get the idea.
Another method would be to set the rowsource depending on the selection.