Can you give us some context for the question? The phrase ?single fault? can be used in many different ways, and I can?t give you an answer without knowing the way in which you are using it. For example, in the nuclear power generation industry, the plants are designed to allow for a safe shutdown with what they call a ?single failure? of any key component. That means you must design the system to be able to operate if one item fails, but you don?t have to design it to operate if two items fail at the same time. The odds of two independent failures occurring simultaneously are too small to be worth consideration.
Is that the context in which you are asking this question (i.e., single failure versus multiple failures)?