Why are two Parashiyot sometimes read on Shabbat?

Since a different portion or Parasha is to read each week, there must be at least one Parasha for every week of the year. Since Hebrew months are lunar, there are 354 days (12 times 29.5) in a 12 month year. In order to keep this lunar month calendar synchronized with the solar year, an additional month (Adar II) is added (approximately once every three years). (Want to learn more on the Jewish Calendar?)

This means that unlike the Gregorian calendar's year with a 52 weeks, the numbers of weeks in a Jewish year can vary, requiring a different number of parashiyot. Leap years may need up to 54 different portions; shorter years only 48. But since the full Torah reading must be completed each year by Simchat Torah, in shorter years (ie., those without the extra weeks), a few portions (typically shorter than usual) are combined into one longer portion. Hence, some years Tazria and M'tzora are separate parashiyot, and in other years joined: Tazria-M'tzora. Combined Parashiyot are termed Parashiyot Mehubarot (Parasha Mehuberet, sing.).

In addition, combined parashiyot are used to keep the Torah readings in Israel and the diaspora calibrated. When the extra 'diaspora' day of a holiday falls on Shabbat, the holiday's specially assigned reading is read in place of the regular of Shabbat reading. But communities in Israel (and most liberal congregations) which don't celebrate the second 'diaspora' day, continue with the regularly scheduled Shabbat reading. The communities that did not read the holiday reading are now 'one week ahead' and continue to read different Torah portions from the traditional diaspora community until a combined parashah is read. Those reading ahead separate the parashah and spread it over two Shabbatot, thereby allowing the diaspora community to 'catch up'.