Is one expiation or two expiations obligatory when breaking two oaths concerning the same thing but with two different wordings?

If a person swears an oath on a single matter and repeats the oath, then breaks it, only one expiation is due. However, if he repeats the oath on different matters and then breaks them, an expiation is due for each oath. Based on this, whoever swears an oath to perform night vigil until he thirsts and until he tires, then breaks both oaths, owes two expiations.