The invention relates to a method for friction stir welding along a butt joint weld line (40). A welding device (70) comprises a probe assembly (90) including at least a first and second probe device (100a, 100b). A first and second part (31a, 31b) to be joined along the weld line (40) are provided, wherein the parts are each formed as a laminate (1; 1') comprising at least a first layer (2a, 2b) formed with a first material and a second layer (5a, 5b) formed with a second material. The first and second materials have different material properties. The method comprises friction stir welding of the parts using the welding device, wherein simultaneously, the first probe device acts on the first layers and the second probe device acts on the second layers. Moreover, the invention relates to a friction stir welding device and to a workpiece comprising a butt weld seam.