Violin soloists take the stage during the orchestra's opening show at the theater.
People are playing the harmonica while standing on a roof.
People are either playing the violin inside the theater or playing the harmonica on the roof. They can't be playing both types of instruments inside and outside at the same time.
The people can be playing a violin or a harmonica. The stage is not likely to be on a roof.
The violin and harmonica cannot typically be played simultaneously, and a theater stage is unlikely to be located on a roof.