Heat 2 tablespoons olive oil in a large skillet over medium-high heat and add sausage. Cook and stir until the meat is browned, about 10 minutes. Pour in wine and bring to a boil; cook, stirring often, until wine has reduced by half, about 10 more minutes. Transfer sausages and liquid to a separate container.
Heat 2 tablespoons olive oil in same skillet over medium-high heat. Cook and stir garlic until fragrant, about 1 minute. Stir in red pepper flakes; cook and stir for 30 seconds. Stir in olives. Mix in fennel, stirring to incorporate. Cover and simmer, adding 1 to 2 teaspoons water occasionally to prevent burning, until fennel is soft, 6 to 8 minutes.
Return sausage and its cooking liquid to skillet and toss with fennel mixture. Cook and stir fennel and sausage mixture until fennel is browned and coated with reduced sauce, 5 to 8 minutes. Serve with a slotted spoon if desired to allow some of the drippings to drain off.
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