Matthew:
Any rule involving more than one property should be applied only at instance validation, not in response to the setting of an individual property (pre- or post-set validation). In other words, the values of the two (or more) properties that are involved in the rule should not be tested for correctness until you are validating the entire object. This permits either value to be set freely, independently of the other; but the freedom ends when the instance is validated in its entirety.
Greg Dunn
IdeaBlade