I join 80/20 with a zig zag stitch but don’t think that would work well with poly. I read a story that said you should do a wave cut to join batting so the quilt doesn’t “ break” along a straight line. I think I would try the wave method and hand stitch a herringbone pattern across the join to connect the batting. Obviously you cannot use the iron on batting tape because the iron will melt polyester batt.