Heat grill to high. Season both sides of fish with salt and pepper and lightly brush with oil. Grill fish until lightly charred on each side, about 1½ minutes per side, it should not be fully cooked. Set fish aside.
Heat oil in a large skillet big enough to hold both fillets over medium heat. Add garlic, olives, and capers. Once garlic begins to brown, add tomatoes and continue to cook for a few minutes. Add broth and bring to a simmer. Add fish fillets and shrimp, cover and cook until shrimp are cooked through, 2–3 minutes. Garnish with fresh basil and serve.