The general case is that an iron that trips the house electrics has developed a fault - usually with the heating element - and the remedy is to throw it away and to buy a new iron.
You do not say if the iron has ever worked - and your scenario is made more complicated by the introduction of the second iron - are they independently failing in separate houses or does it take both of them in the same property to cause the trip? I assume the former in which case then it's a coincidence of similar failures.
