I have quilted a denim quilt with plain fabric on the back and it was heavy. I did a basic straight stitch down the quilt in rows, alternating directions. I used my old singer treadle because I knew my modern machine would not sew over the welted side seams and pockets I had added. Are you using denim like whole yardage (not pieces)for the back? If you are, you will probably be okay to do very basic rows of stitching although it will be very heavy to work with.