Heat 1 tablespoon of the oil in a large nonstick skillet over medium-high heat. Add the pancetta and saute until crisp. Transfer to a paper towel-lined plate. Add the remaining 1 tablespoon oil to the skillet along with the garlic and shallots and cook, stirring occasionally, until softened and golden brown. Fill the skillet with the mustard greens (as many as will fit), sprinkle generously with salt and pepper and continue cooking until wilted, just a couple of minutes. As the greens cook, add the remaining greens, lemon zest and juice to the skillet, cooking until the greens are completely wilted. Remove from the heat, transfer to a warm serving platter and top with the crispy pancetta. Serve right away.