These are my favorite three versions of the famous Cat Duet, perhaps by Rossini.
First we have Elisabeth Schwarzkopf and Victoria de los Angeles. You can't really tell their voices apart. On second thought maybe you can.
The next one is more your grand divas Leontyne Price and Regine Crespin.
The last one is quite bizarre: Nicolai Gedda and Frederica von Stade. I think they're the funniest.
I apologize for the lack of videos. Oh what the heck. Here's another one with pictures. In spite of the German narration, this is obviously a Prom.