Mike, it maybe that one of them has tripped as this will happen if it overheats and automatically shuts off. It is possible for just one to switch off if it is getting the water after the first exchanger - as it recieves hotter water than the first, this will then overheat quicker if left idle and no cleaning is going on.
There is a very small reset button near the power leads leading into the actual heat exchanger.
I havent checked myself whether one gets hotter than the other, but logically speaking I think this would happen as they are inline, and one has to get the water first before it leads to the other - this last one would be the hotter of the 2.
Hope this helps
Kev