Ok, if I follow what you're saying you want the update to happen after both combo boxes are filled out. This solution assumes you're using VBA and that the combo boxes are empty until they choose something. I made a quickie form with 2 combo boxes (0 and 2) and put a subroutine call in each of them. Then in the subroutine I check if either is empty and exit if so. If not, it continues on to do stuff:
Code:
Private Sub Combo0_AfterUpdate()
Call CheckThenUpdate
End Sub
Private Sub Combo2_AfterUpdate()
Call CheckThenUpdate
End Sub
Private Sub CheckThenUpdate()
If Nz(Me.Combo0, "") = "" Or Nz(Me.Combo2, "") = "" Then Exit Sub
'do update query here
End Sub