I used to overlap the two pieces slightly, then use a rotary cutter to make a wavy line through them, then use needle and thread to whip the two pieces together - no overlap. However, last time I butted the two pieces together and used the thinnest fusible interfacing to join them. I cut the interfacing into 1.5" strips, laid it on the batting where the two pieces came together, laid a strip of cotton on top of that, and pressed with a hot iron. I did it on my cutting table, since it is much longer than my ironing board. I have that batting in a quilt on the longarm now, and the seam is hanging tight and close. I will use this method again next time. (This is for cotton batting, not for poly.)