I use the following to check for both conditions, a null or zero length string ...
Len(Nz(Me.txtControlName1, "")) = 0
So you might be able to use something like ...
If Len(Nz(Me.txtControlName1, "")) = 0 And Len(Nz(Me.txtControlName2, "")) = 0 Then ....
HTH,
-dK