As you say, Mathilde Gravel (Ignace-Christophe - Euphrosine Doyon) was born and baptised on 13 November 1846 - at St. Barthelemy, Berthier County, Quebec. Source: parish records of St. Bartholemy Catholic Church (civil records copy), St. Bartholemy, Berthier County, Quebec; page 31 in 1846, baptismal record #85 in 1846; available in ancestry.ca's Drouin data base (contains a scanned image of actual record). She married Edmond Belair in 1862.
Regarding the second Mathilde, it is doubtful that she was born to Luc Gravel and Mathilde Gauthier. I can tell you that her birth/baptismal record is not at Ste. Agnes, Charlevoix County, Quebec, where the couple married and where they resided most of their life. If Mathilde existed, she was likely dead by 1851, as the couple is shown in the 1851 Canadian Census with no children in the family group.
I do have a Marguerite-Domathilde, born 14 September and baptised 15 September 1846 at St. Martin, Laval, Ile Jesus, Quebec to Augustin Gravel and Marguerite Laurin. Source: parish records. She married John Jubinville in Chicopee, Massachusetts in 1867.