I was watching Sewing with Martha (I think that's the name of the program???) and her guest was FM quilting using a double/twin needle. She said you don't have to do anything special other than read your owner's manual on how to thread your machine for two spools/threads. And she warned the back side of double needle stitching looks like a zig-zag and that should be taken into consideration- but it could be somewhat hidden with a busy print for the backing. But it opens the door for some new and interesting possibilities!
I can see using a double needle with a wide space between the needles and then threading a strand of yarn between the two rows of stitching and between the quilt top and batting for a trapunto effect. In Celtic quilting patterns it could be stunning. Or taking a step out of echo quilting since you could stitch two parallel rows at once. Or outlining a feather quilting motif with a double row of stitches that are perfectly spaced. Or even quilt the feather with two rows of stitches. Or rather than stitch in the ditch you could straddle the seam and have perfectly parallel rows on each side of the seam line. Or stitching down raw edged appliqué with the double row of stitching Lots of good stuff when you start thinking...
Just an idea I thought somebody might like to grab and run with it.
Leslie & The Furbabies in MO.