<p>if we simplify the first one, then:
blah, blah, and blah are forms of dance; the dancers are professionals.</p>
<p>taking out the second “are” does not work:
blah, blah, and blah are forms of dance, the dancers professionals.</p>
<p>even if we keep the “performing” part it still does not make sense:
blah, blah, and blah are forms of dance, the dancers professionals performing for blah.</p>