In the 1834 census, it says that son Johann Georg Fritzler was Michael Fritzler's son by his first wife, implying that Michael was married two times. Since his oldest child Johann Georg was born in 1802, he must have been Katharina Grimm Fritzler's child; he'd married her by 1798. A note next to younger son Johann Adam's name says he was Michael Fritzler's son by his second wife. This indicates that his first wife was probably deceased, especially since he would not have continued to live with his father-in-law if the couple had divorced. Michael Fritzler's youngest son -- born to his second wife -- was born in 1815. This means his first wife passed away by 1813 and he had remarried his second wife, also named Katharina, by 1814.
To sum up:
Michael Fritzler's first wife was Katharina Grimm, daughter of Adolph Grimm, born about 1780.
Michael Fritzler and Katharina Grimm were already married at the time of the 1798 Grimm census but had no children yet.
Michael's first child by Katharina Grimm was son Johann Georg Fritzler, born about 1802.
Michael Fritzler's second wife was Katharina, last name at birth unknown, also born about 1780.
Michael and his second wife Katharina had their first child in 1815, so they must have married no later than 1814. This means his first wife passed away at some point before 1814.
Michael continued to live with his first father-in-law after his remarriage to the second Katharina.
A comment in the 1834 census suggests that Michael's second son, Johann Adam, could have been Adolf Grimm's [biological] grandson. This is impossible if his mother was not first wife Katharina Grimm. He was most likely Adolf Grimm's step-grandson.
Head of the Household Adolph Grimm, age 63 in 1816, deceased 1828
Widower of Child #1 Katharina Grimm Michael Fritzler, age 41 in 1816, deceased 1827
Grandchild #1 Johann Georg Fritzler, age 32, Michael Fritzler's son by first wife
Wife of Grandchild #1 Katharina Fritzler, age 31
Great Grandchild #1 Maria Magdalena Fritzler, age 14
Great Grandchild #2 Maria Elisabeth Fritzler, age 10
Great Grandchild #3 Katharina Margaretha Fritzler, age 7
Great Grandchild #4 Johann Jakob Fritzler, age 5
Great Grandchild #5 Johann Konrad Fritzler, age 3
Great Grandchild #6 Anna Margaretha Fritzler, age 6 months
Second Wife of Michael Fritzler Katharina Fritzler, age 54
Grandchild #1 Johann Adam Fritzler, age 19
↑The 1775 and 1798 Census of the German Colony on the Volga, Lesnoy Karamysh, also known as Grimm; Published by the American Historical Society of Germans from Russia, Lincoln, Nebraska, USA; Published date: 1995; family #1 in the Appendix of the 1798 census.
↑1834 Census of Grimm in the District of Saratov, Russia, dated 2 February 1835; Translated by Brent Mai, Concordia University, Portland, Oregon; Published by Dynasty Publishing, Beaverton, OR, USA; Published 2011; page 76, family #259.