Another thing the mixing, and mastering for that matter, may request/ prefer (you can never have enough options) is a stem mix.
Stems are sub groups of instruments. For example, say you have created/ recorded a kick track, snare track, hit hats etc etc.... these can all be bussed to an Aux track which in turn would make a Drums channel. Same for vocals and backing vocals, guitars, bass, and effects.
When everything is stemmed, the mix engineer doesn't have to work on a micro level and yet he/ she still has control of the blend and overall mix process.
Depending on how good a job the mix is will depend on how much work the mastering engineer has to do for final polishing. Some mastering guys prefer access to the stems the mix engineer ends up with. 9 times out of 10 though, if the mix is good, the mastering is really only tweaking aesthetics of the mix to work with other tracks in an album.... and that task is perfectly fine to do on stereo tracks.