Jussi Björling and Renata Tebaldi in the '50s singing most of the final scene of Act 1 of La Bohème:
Björling in "Che Gelida Manina":
Followed by Tebaldi in "Mi Chiamano Mimi", and the duet that concludes Act 1:
I'm not sure where exactly this clip comes from. One thing about the way they're recorded here is that, unlike more high-tech audio and video recordings, you really get a sense of the size of the voices -- Tebaldi's voice was huge and the microphone sometimes sounds like it's having trouble recording it; Björling's voice, while it never had any trouble projecting, was smaller than hers and sometimes gets drowned out when they're singing together.