You could arrange the thicker thread on the top and then couch it down by machine (zigzag stitch) with invisible thread (invisible in the top, neutral thread like Bottom Line or Masterpiece in the bobbin)

or use water soluble thread (top & bobbin) to do the stitching with the 12 weight. Then when you wet it to block it the water soluble will come right out no ripping needed.

I was kind of thinking that's what I was going to have to do. Makes it a bit harder when this is an art quilt and I was going to use the striations in the fabric to sew around. I guess I could use a basting stitch and do it first, the flip it and do the 12wt in the bobbin. Hmm, will have to think more on this. Thanks for the help though

12 weight belongs in the bobbin. It's really too heavy for the top of the machine. Wind it onto the bobbin. Adjust the bobbin tension so the thread flows well and do a Ricky Tims and mark the quilt from the front to the back and have at it. Use either Bottom Line or So Fine in the top.

I have wanted to do my freemotion embroidery quilting with a 12weight thread, it's a sulky 100% cotton varigated. I started testing it and I can't get it to grab the bobbin thread and then if I get a few stitches, the thread breaks. I have tried changing tension, needle (started with a 100/16 jean needle (now have a 90/14 topstitch in) bobbin thread, but I can't get it to work. What do I need to do?? The machine works fine with regular 40 weight.