Bring a large saucepan of salted water to a boil. Add orzo and cook according to package directions until al dente. Drain in a colander and set aside.
While pasta is cooking, melt 1 Tbsp. butter in a large skillet over medium heat. Add diced pancetta and cook, stirring, until crisp, about 6 minutes. Transfer with a slotted spoon to paper towels. Add squash and zucchini to pan and cook over medium-high heat, stirring occasionally, until golden and just tender, about 4 minutes.
Add cooked orzo, tomatoes, arugula, salt and remaining 1 Tbsp. butter to pan with zucchini; stir gently until arugula becomes wilted. Stir in chopped fresh parsley. If desired, serve with finely grated Parmesan cheese.