Sometimes two relevant historical singers of a given character perform together, but one of them in a still minor role.
Perhaps the best known example is the 1952 Covent Garden Norma, with Maria Callas as Norma, and Joan Sutherland as Clotilde:
But in this post, I would like rather to comment on Traviata, and a performance in Vienna, back in 1971, with Ileana Cotrubas being Violetta, and Edita Gruberova singing Flora.
This is a moving performance by Cotrubas, with all her strengths and weaknesses as Violetta, somewhat superior to her later studio recording with Kleiber and Domingo, in my view. A couple of examples:
Funny enough, neither Sutherland would sing years later Norma in a way resembling Callas (but both were great), nor Gruberova will take anything from Cotrubas's Violetta (though both were very good, but not great, Violettas):