Hello Bill,
Forgive me if you are already familiar with this process, but let me ask...
What is your final product in this case?
I would imagine you are speaking of a stereo music mix, rather than a surround mix or a film soundtrack, where the use of "stems" is more common...
In the "traditional" world of mastering, the engineer usually works with just the 2-track mixdown...
However, in the past 10 years or so, as the job of mastering has become more "workstation oriented," producers and engineers and A&R types have had the opportunity to put off decisions still further...(g)
Thus, we have seen more and more vocal/instrumental "split stereo" mixes delivered to the mastering studio. (vocal mix on 2 tracks, instrumental mix on 2 more tracks.)
If there is a "standard" for split mixes, I suppose this is it. Perhaps a very complex song may call for the backing vocals on their own stereo pair,giving the ME six tracks to work with...or perhaps this just shows a lack of confidence on the part of the producer?...(sorry.)
Keep in mind that it is NOT the job of the mastering engineer to MIX your song for you. (Although I can certainly do so, you may not get what you want out of it.)
You may be better served by mixing several versions of each song, and then working with the mastering engineer to choose the best one. Don't make it any more complicated or expensive than you have to...
best of luck,