Toast pine nuts in a heavy skillet on medium heat until fragrant and golden brown. Repeat process for pecans. Combine the pine nuts, pecans, garlic, and cilantro in a food processor. Pulse until fine and well mixed. With food processor running, slowly add oil. Mix a few times by hand to thoroughly combine pesto. Add parmesan cheese and salt (optional).
Spray olive oil cooking spray or lightly rub olive oil onto grill grate to prevent sticking. Preheat grill to medium heat. Cut squash lengthwise (about three pieces per squash depending upon size). Make 3 or so hatch marks on cut sides of squash and rub pesto onto cut sides of squash. Grill squash for about 5 minutes on each side or until golden grill marks appear and squash is tender. Remove and serve with extra pesto on the side if desired.