Family #114 in the 1775 Grimm census.
Family #23 in the 1798 Grimm census.
Family #54 in the 1834 Grimm census.
Note: at the time of the 1798 census, he was a widower. The name of his deceased wife is not given. He and this deceased wife had two children: Johann Konrad and Anna Margaretha. The 1834 census clearly states that he had children by three different wives. The names of two of those wives are unknown. So yes, having two unknown spouses attached to him is correct, in addition to his third wife who is named in the 1834 census. Here is a list with children separated by name.
Unknown first wife's children
Johann Konrad, born 1797
Anna Margaretha, born 1796
Unknown second wife's children
Johann Kaspar, born 1801
Children of third wife Anna Barbara
Johannes, born 1807
Maria Katharina, born 1812
Johann Georg, born 1816
Konrad's first wife was deceased before 1798, possibly in connection to complications from the delivery of her second child. Konrad remarried by 1800 to his second wife; her first child was born in 1801. This second wife passed away by 1805, he remarried his third wife by 1806, and the first of their three children was born in 1807.
Head of the Household Kaspar Kimel [sic] [Kimmel?], age 37
Wife Johanna Maria Kimmel, age 38
Child #1 Alexander Kimmel, age 7 years 6 months
Child #2 Adam Joachim Kimmel, age 3 years 6 months
In the 1798 census, he is called Johann Andreas, rather than Alexander. Generally speaking, the 1798 census is more accurate than the 1775 census. It catches full names, LNABs for women, etc. His age matches that of Alexander, and he is exactly four years older than his younger brother. His parents didn't have another child, and it is unlikely that a younger child in 1775 would have been omitted from the census. I think the census taker may have incorrectly taken the wrong name down for him. For now, I am going to change his name to Johann Andreas and leave Alexander as an alternate name.
Head of the Household Andreas Kimmel, age 48 in 1816, deceased 1831
Wife Anna Barbara Kimmel, age 57
Child #1 Johann Konrad Kimmel, by first wife, age 26 in 1816, to household #30
Child #2 Johann Kaspar Kimmel, by second wife, age 15 in 1816, to household #269
Child #3 Johannes Kimmel, by third wife, age 27
Child #4 Maria Katharina Kimmel, by third wife, age 22
Child #5 Johann Georg Kimmel, by third wife, age 18
↑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 #114 in the 1775 Grimm census, Kaspar Kimmel family.
↑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 #23 in the 1798 Grimm census, Kaspar Kimmel family.
↑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 17, family #54, Andreas Kimmel family.