I have used 2 strands of regular 50/30 wt thread for buttonhole stitch machine applique. Have never tried double strands for quilting. You may need to reduce the upper tension on your machine. Be sure to make a practice piece, as you may also find that you need to quilt more slowly so that the two threads are always closely aligned. When I worked with 2 threads, I used separate thread spools but treated the threads "as one" from there.