Does 1 string bass = 2 cellos? If so, in general terms, when would you want to break this "rule"?
I am writing for a small pit orchestra with one string bass and two cellos. I sometimes encounter a situation where I want to have the cello line doubled by the string bass an octave below (cello and bass parts being played arco). Almost as frequently, I have a bass line that alternates between cello & bass in unison and divisi (again, cello and bass arco). In these cases, do I want to have two cellos on the cello line or just one? What will balance best? I am looking for a blended sound here, where the individual instruments are not easily distinguished. Just a nice, solid support for the higher instruments that doesn't call much attention to itself.
Elsewhere in the score, the bass is played pizz and functions more like part of the rhythm section and the strings are divided in one of two ways: 2 part harmony with unison violins over unison cellos, or unison violins over divisi cellos (with the first cello playing what would have been the viola line.) This seems to be working quite well.
But I hear little difference between using one cello or two with the bass as described above. I am not sure that I can trust my software because it is just too easy to adjust the volumes of the virtual instruments to make them balance (especially when I am using samples from different libraries that were recorded at different levels and some compensation is needed in any case.)