Why in Israel do families celebrate one Passover Seder and in North America Jews celebrate two seders?
In ancient times, the beginning of a new lunar month had to be determined by direct observation of the new moon. Among Jews, the only observation that was "official" was the one certified by the authorities in Jerusalem. This was necessary to make sure that all Jews observed the same calendar dates.
However, many Jewish communities, including the large Jewish community in Babylon, could not reliably get word from Jerusalem about the day of the new moon before the holiday began on the fifteenth day of the month. For this reason, Jewish communities outside the land of Israel adopted the practice of observing an extra day of the pilgrimage holidays (Passover, Shavuot, Sukkot and Sh'mini Atzeret), just in case they had gotten the date of the new moon wrong.
This practice for Jews outside the land of Israel continued even after mathematical models made it possible to calculate the date of the new moon. It is still the practice of Orthodox and Conservative Jews living outside of Israel. The practice has nothing to do with providing an extra seder to allow couples to spend one night with each set of in-laws!
In the 19th century, the Reform movement abolished this practice of "Yom Tov Sheni," "The Second Day of the Holiday," because the reason for its creation no longer existed. For this reason, Reform Jews -- inside and outside the land of Israel -- usually observe only one seder.
There are many "midrashic" explanations for the second seder. They make great sermons, but the historical explanation follows the outline I've given you here.
Rabbi Jeffrey W. Goldwasser