If you want to do this programmatically, in the Before Update event of a
form, rather than in a table-level validation rule, it would look something
like ...
If IsNull(Me!FirstTextBox) And IsNull(Me!SecondTextBox) Then
MsgBox "Please enter a value in at least one of the text boxes."
Cancel = True
End If
If you want to do it in a table-level validation rule, it would look
something like ...
(FirstField Is Not Null) Or (SecondField Is Not Null)
To define a table-level validation rule (not to be confused with a
field-level validation rule, which can not refer to any other field in the
table) select Properties from the View menu while in table design view.