As many of the replies so far indicate, the general purpose of equipping a preamp with two volume controls is to negate the requirement for a balance control and not to provide control over two different outputs.
To achieve the latter, assuming the preamp is not purely passive, would require duplication of a large portion of the preamp's circuitry - which would not make commercial or sonic sense.
As regards the original post, the complexity of the speaker arrangement suggests the possibility that there may be frequency response issues that cause some recordings to have too much bass and others not enough.
The Jadis JP 80 MC and some Croft Preamps also have/had independent volume controls - again for balance adjustment purposes rather than to control discrete outputs.