Most supporting bands are there to support the main band. They wind up the crowd; they get the crowd revved up/enthusiastic/ in the mood. They are, by definition, a support band, and not the most famous of the 2 bands. They are less well known, less famous, and usually less talented, less artistic, etc. So it would be unusual for them to upstage/out-perform the star. They would have to be super-talented, with much better music or acts, rhythm etc to outperform/upstage/out-shine the star, which is the main band.