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.