to get edges that fit together exactly, overlap the two pieces a little and rotary cut down the double thickness - get rid of the two "leftover" scrappy pieces, butt the big pieces together and zig zag them together. Trying to butt together edges that are not cut this way can lead to a distorted seam that might be noticable. The cut you make thru the double thickness doesn't have to be straight as long as you cut both pieces the same they'll fit together.