one's spouse's brother-in-law; that is, one's sister-in-law's husband (one's spouse's sister's husband), or, the brother of one spouse in relation to the siblings of the other spouse.
Miguel Muliart (concuñado de Colón, casado con Briolanja Muñiz, hermana de su primera mujer)
"Michael Muliart (concuñado of Columbus, married to Briolanja Muñiz, sister of his first wife)" — Consuelo Varela & Isabel Aguirre (2006:28) La caída Cristóbal Colón
"[Concuñado] literally means "co-brother-in-law" (fem, "co-sister-in-law") and refers to Ego's spouse's sibling's spouse. In criollo usage, however, the brothers-in-law of a concuñado become Ego's concuñado as well, and this usage is extended indefinitely. The concuñado relationship is chiefly of importance between men; little stress is placed upon it among women, or between women and men." — Dwight Heath & Richard Adams, Contemporary cultures and societies of Latin America: a reader in the social anthropology of Middle and South America and the Caribbean, ed. 3, 1965
(in the plural) The relationship between two people who marry siblings: Men whose wives are sisters, a man and woman whose wife and husband are brother and sister, etc.
Pelea entre concuñados: […] Una riña entre los novios de dos hermanas terminó de la peor manera.
"A fight between concuñados: ... A fight between the grooms of two sisters ended for the worst." — Los Andes on line, 4 April 2009
one's brother-in-law or sister-in-law's brother; that is, one's sibling's spouse's brother (one's sister's husband's brother or one's brother's wife's brother).
(in the plural) The relationship between two people whose siblings are married to each other: Men whose brother and sister are married, etc.