I can't find one that permits it, but also can't find one that prohibits this either. Am suggesting that it is a similar situation to the OP. IMO if a conductor is not going to carry current at the same time as another conductor then the heat produced within a raceway would be the same as if the non carrying conductor were not there which is the intent of the deration to begin with. I can see some arguement to this but if there would be some type of interlock to keep both loads from operating at the same time then why not? A similar situation exists for feeder and service calculations as well as neutrals carrying unbalanced current. The real question is what are current carrying conductors for this particular situation?
actually, a similar does not exist for feeders and service conductors, because they are allowed to be smaller for non coincident loads - see 220.51 and 220.60. IMSHIO, derating for noncoincident conductors is ridiculous.