Mix the mayonnaise with the garlic in a small bowl and set aside. Using your hands, mix together the turkey and turkey sausage in a large bowl until just combined. Form this mixture into four 1-inch-thick patties.
Preheat a large skillet over medium heat. Add the olive oil and pancetta slices and cook the pancetta on both sides until crisp, about 3 minutes. Drain on paper towels and reserve.
Place the same skillet over medium-high heat and add the burgers. Cook, turning once, until cooked all the way through, about 5 minutes per side. Top with the fontina and cook until just melted.
To serve, place the burgers on the focaccia bottoms. Top with tomato slices, pancetta and arugula. Spread the focaccia tops with garlic mayonnaise, set on top of the burgers and serve.