Adjective + verb ing

'Working' in 'She's busy working [three jobs]' is a complement of the adjective 'busy', not a type of second predicate. In The Cambridge Grammar of the English Language (Huddleston & Pullum; 2002) (p 1259):

Gerund-participial complements

The adjectives busy and worth/worthwhile license complements of this form:

She was busy preparing her report.

These objections aren't worth bothering about.

It isn't worth taking the matter any further.

CGEL gives further analysis of the 'worth / worthwhile' examples.

..........

Contrast 'She's very tired, [what with] working such long hours'

and 'She's very busy, [what with] doing three jobs'

which use adjuncts, here (ignoring the what with's) participial clauses of reason (or expansion perhaps, in the latter example).