Generally a Co-principal is expected to be able to fill-in with the same consistency and quality as a Principal, in the case that the Principal cannot perform his/her duties or has been asked away. A lot of orchestras, especially in Europe it seems, have two orchestras functioning differently, such as the Paris Opera where there is a chamber group that does stage concerts, and the full opera orchestra that is in the pit. In this case there are two principals (or "soloists"), on leading the bass section of each of the split orchestra. In the USA, a co-principal is not as common as an Assistant Principal. An assistant's duties are similar to a co-principal: he or she must perform the principal duties at times when the principal cannot. But an assistant is also expected to carry the "lead" of the principal to the rest of the section (as does a co-principal). One thing that truly separates the co-principal position from the assistant position is the amount of pay. Usually the co-principal gets very close to the same salary as the principal. An assistant rarely gets that much, but still more than the section players. So, it is financially better to play co than assist. Basically, the co-prin is the same as principal, with duties being expected when the principal is not available.