Preheat oven to 400°F. Pierce sweet potatoes with fork. Place on rimmed baking sheet or in roasting pan and bake until almost soft, about 40 minutes. Add bananas to roasting pan alongside sweet potatoes and bake until banana skins brown, about 10 minutes. Let sweet potatoes and bananas stand at room temperature until cool enough to handle, about 15 minutes (maintain oven temperature). Cut sweet potatoes in half. Scoop flesh from sweet potatoes into large bowl. Peel bananas; add to bowl. Add 1/2 cup butter and honey; beat with wooden spoon until blended and fluffy. Season to taste with salt. Transfer to 8- to 10-cup ovenproof baking dish.
Mix nuts, sugar, and flour in bowl. Add 1/2 cup butter; mix with fingertips until coarse crumbs form. (Potatoes and streusel can be made 1 day ahead. Cover separately; chill. Before continuing, preheat oven to 400°F.) Sprinkle streusel over potatoes. Bake until top is golden brown and potatoes are hot, about 25 minutes.
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