Use the BeforePrint event in the ThisWorkbook module. Are you familiar with
this?
Here is some sample code:
Private Sub Workbook_BeforePrint(Cancel As Boolean)
'BOTH cells must contain data before printing-
If Application.WorksheetFunction.CountA(Range("A1,C25")) < 2 Then
Cancel = True
MsgBox "A1 and C25 must have values"
End If
End Sub