If the artist you're collaborating with is planning to contribute musically, then send the whole session. Hopefully he uses the same DAW and has the same virtual instruments. There is benefit to him being able to mute certain tracks, tweak instruments, change melodies and groove, etc.
If the artist you're collaborating with is just going to provide vocals, then a two-track WAV or mp3 is fine. He can make a new session with your two-track and his vocal tracks. Then he should send his multitrack session or a bounced version (uncompressed WAV) of each vocal layer for you to incorporate into your full mix.