Heat a large skillet over medium heat and spray with olive oil.
Add the ground sirloin and ground Italian sausage to the skillet and cook for 7-10 minutes, stirring occasionally, until cooked through.
Drain any fat from the meat, add it back to the skillet, and stir in the garlic. Cook for about a minute until the garlic is soft.
Stir in the marinara sauce and season with salt and pepper, to taste. Simmer for 1-2 minutes more, remove from the heat, and set aside.
Preheat the grill to medium heat.
Slice the zucchini in half, lengthwise. Use a small spoon to scoop out the seeds and hollow out the center. Spray the zucchini boat shells with olive oil spray and season with salt and pepper.
Grill cut side down 3 to 4 minutes on medium direct heat.
Remove the zucchini boats to a baking sheet, fill with the beef mixture, sprinkle with shredded cheese and carefully place them back on the grill using a wide spatula, close the cover and cook for 3-4 minutes more until the cheese is melted.