You can use machine basting to keep abutted seam edges together while applying underlays. This is most commonly used when sewing slot seams.
A machine basting stitch is the longest stitch available on your sewing machine.
Machine baste the seam as you would stitch any seam
Clip every 5-6 threads.
gently iron open.
After applying and topstitching the underlay, you can easily pull the seam apart