There's no rules for opening acts. For example when Patti Smith opened for U2 at Madison Square Garden, they gave her forty minutes
And when Judy Garland played the Palace in New York She had Two opening acts 1967. At her Christmas concerts at Madison Square Garden in New York, Judy had guests like Tony Bennett and Liza Minnelli
She had. no opening acts at Boston Garden in October 1961.
However she was pure magic that night