Nazir Chapter 3, Mishnah 2. This mishnah builds on the principle taught in the previous mishnah: someone who takes an unspecified nezirus vow must complete his nezirus on the thirty first day, and after the fact, if he completed it on the thirtieth day, it is valid. Our mishnah expresses that very same idea, only it applies it to a case of two consecutive periods of nezirus, one after the other.
Ideally: shaving on the thirty first day and on the sixty first day
Someone who vows to be a nazir twice shaves for the first nezirus on the thirty first day, and for the second on the sixty first day. That is the ideal time. The reason, as the mishnah itself explains at its conclusion: on the thirty first day, once he has shaved and brought his offerings, the second nezirus begins on that very day. And since the second nezirus begins on that day, its thirtieth day is day sixty, and therefore the shaving and the offerings should ideally be done on the sixty first day.
When he shaved for the first nezirus on the thirtieth day (valid after the fact)
The ideal time for the second nezirus: day sixty. If he did the shaving and the offerings on the thirtieth day, which is valid after the fact as we learned in the previous mishnah, then the first day of the second nezirus was the thirtieth day of the first nezirus. It turns out that the day that would seem to be its first day is really its second day, and therefore day sixty is truly the thirty first day of the second nezirus.
After the fact: day fifty nine. If he shaved for the second nezirus on day fifty nine, it is valid, since that is the thirtieth day of the second nezirus.
The testimony of Rabbi Papus
The mishnah concludes with the testimony given by Rabbi Papus: someone who vowed two periods of nezirus and shaved for the first nezirus on the thirtieth day, which is the time that is valid after the fact, so that this day in fact became the first day of the second nezirus, should ideally shave for the second nezirus on day sixty, which is really its thirty first day. And if he shaved on day fifty nine, he has fulfilled his obligation, since that is really its thirtieth day.
In summary: the principle at work in this entire calculation is that the thirtieth day of the first nezirus does double duty and counts as the first day of the second nezirus. On the strength of this principle the times for shaving were set: ideally the thirty first day and the sixty first day; and for someone who shaved for the first nezirus on the thirtieth day, ideally day sixty, and day fifty nine after the fact.