To the left is a typical symphonic wind section, where musicians double on other instruments.
To create a secondary instrument, simply drag an instrument from the instrument library (or from another musician) to the musician in question.
You can also option-drag instruments and musicians to clone them. If you look closely, there are some subtleties involved in how the musicians are numbered.
Note particularly the numbering of the clarinets: Igor knows how to distinguish between various transpositions of the same instrument and numbers everything accordingly.
