Preheat gas grill or prepare charcoal BBQ for grilling. Season steak with salt and pepper on both sides.
Slice zucchini and squash lengthwise and spread on platter. Brush lightly with olive oil and season with salt and pepper.
Combine parsley, cilantro, garlic, capers, vinegar, chili flakes, 1/2 teaspoon salt, and 1/2 teaspoon pepper in small mixing bowl. Add approximately 2 1/2 tablespoons olive oil, mixing well to combine. Leave at room temperature for flavors to meld.
Grill steak approx 6 minutes per side depending on desired temperature. Remove from grill and allow to rest while you grill the squash, which takes about 2 minutes per side.
Slice steak against the grain and serve alongside zucchini drizzling both with chimichurri.
Chimmichurri can be made several hours ahead, cover and leave at room temperature.