The Integrated Tariff Configuration rulesets include a section describing Invalid Currency Combinations. 

Invalid Currency Combinations are described in terms of If Currency A and Currency B then Currency C.  For a given episode or visit each set of Currency Charge Records are checked agains the possible Invalid Currency Combinations if there is an invalid combination the replacement Currency is inserted.  In this case if both Currency A and Currency B have been triggered for the episode or visit then Currency C replaces them both, the Currency Charge Records are updated accordingly.