Begin by filling in both grids (as in a standard Siamese crossword, each pair of clues gives two answers in some order for the two grids).
For six of the across clues, this answer to one clue of the pair of clues is a "doubler" word; a word which consists of the same string doubled twice. These clues are
8-Across (Type of fish): MAHI MAHI
15-Across (North African dish): COUSCOUS
37-Across (Cheerleader's implement): POMPOM
39-Across (Important people): HOTSHOTS
61-Across (Polynesian island): BORA BORA
63-Across (Cream of ____): TARTAR
These clues should be entered in both grids, with half going in each grid (highlighted in yellow int he above grids). The remaining unused clues are in fact words that appear elsewhere in the grids.
8-Across: PIER -> STAR
15-Across: COP -> TOP
37-Across: IRON -> AWOL
39-Across: TRICK -> RELET
61-Across: GOAL -> EDAM
63-Across: UPON -> SPAM
The first letters of the corresponding words in the opposite grid spells out the answer, which is STARES.