Slice your zucchini and squash into circles 1/3 to 1/2 inch thick. (I would not go any thinner than 1/3 inch.)
Preheat your grill to medium heat.
In a large bowl add your vegetable slices, olive oil, and seasonings. Use your hands or a spoon to move around the veggies until they're lightly coated.
Lay zucchini and squash slices on the grill. We'll use any remaining oil at the end so just leave it in the bowl. Grill for 4 to 5 minutes per side (8 to 10 minutes total) or until you achieve the desired softness.
Remove from grill and plate. Drizzle remaining oil/herb/seasoning mixture over your grilled zucchini and squash.
Notes
You can of course use all zucchini or all yellow squash in this recipe if you prefer.