Line a large rimmed baking sheet with paper towels. Cut 2 halved zucchini and 2 halved small summer squash into 2-inch pieces; place flesh-side up on the prepared baking sheet. Sprinkle with ⅛ teaspoon salt; let stand until water leaches out, 8 to 10 minutes. Pat dry with paper towels.
Heat 1 tablespoon oil in a 12-inch skillet over medium-high heat. Add 2 lemon halves, flesh-side down; cook, undisturbed, until charred, 3 to 4 minutes. Remove from the pan.
Heat the remaining 1 tablespoon oil in the skillet over medium heat. Add the zucchini and squash, flesh-side down; cook, undisturbed, until browned, 3 to 4 minutes. Flip and cook until fork-tender, about 2 more minutes. Sprinkle with ⅛ teaspoon salt; transfer to a serving platter.
Add ½ cup wine, 3 sliced cloves garlic, ¼ teaspoon crushed red pepper and the remaining ⅛ teaspoon salt to the skillet; cook over medium heat, scraping up any browned bits on the bottom of the pan, until reduced by half, about 2 minutes. Add 5 tablespoons cubed butter; cook, stirring constantly, until well incorporated, about 2 minutes. Remove from heat.
Spoon the sauce over the squash. Squeeze the charred lemons over the top; sprinkle with 1 tablespoon parsley. Garnish with crushed red pepper, if desired.