Preheat oven to 450°. On a large baking sheet, place pizza crusts. Spread 1⁄2 cup pizza sauce over each crust. Sprinkle each with 1 package mozzarella. Set aside.
In a large skillet, heat olive oil over medium-high heat. Add sausage; cook, stirring occasionally, until browned, about 5 minutes. Using a slotted spoon, remove sausage from pan; let drain on paper towels. Add bell peppers and onion to skillet. Cook, stirring occasionally, until just tender, about 5 minutes. Using a slotted spoon, remove peppers and onion, reserving drippings in skillet. Arrange sausage and pepper mixture over pizzas. Bake 8 minutes.
Sprinkle shrimp with Cajun seasoning. Heat reserved drippings in skillet over medium-high heat. Add shrimp; cook just until pink, 1 to 2 minutes per side. Arrange shrimp over pizzas; sprinkle each pizza with 1⁄2 cup Havarti. Return to oven; bake 2 minutes more. Top with green onion.