Make the validation like this:
($Entity/Field1 = empty and $Entity/Field2 = empty) or
($Entity/Field1 != empty and $Entity/Field2 != empty and $Entity/Field1 <= $Entity/Field2)
This can be easily achieved with a couple of decisions.,
If $Entity/Field1 = empty and $Entity/Field2 = empty then proceed then check if If $Entity/Field1 != empty and $Entity/Field2 != empty.