In a large skillet, melt 2 tablespoons of the butter. Add the porcini and shiitake and season with salt and pepper. Cook over high heat, stirring occasionally, until the shiitake are softened and lightly browned, about 8 minutes. Add the shallot and garlic and cook until fragrant, about 2 minutes. Add the porcini soaking liquid, stopping before you reach the grit at the bottom. Boil until the liquid is evaporated, about 5 minutes. Scrape the mixture into a bowl.