Installed on a crochet galloon machine is a device (1) for inserting alternately-interposed wefts, comprising two weft-inserting sliders (10, 16) carrying respective threading tubes (11, 17) each adapted to lay down a weft yarn (12, 18) to be interlooped in the article of manufacture (M) being worked. Each slider (10, 16) is movable along a respective weft-inserting guide (9, 15). The cyclic movements of the sliders (10, 16) are mutually offset according to a cycle fraction equal to the number of the provided weft-inserting bars (9, 15), so as to alternate laying down of each weft yarn (10, 18) with each chain stitch (6b)-formation cycle.