honest question and dumb answer, so bear with me and my stolen-from-the-internets non-TOJ photos, but:
single needle diamond quilt is akin to this: (disregard actual size of cells, stitch spacing, and square/diamond shape)
double needle quilt has double seams where the single needle has single seams. My intent here is to run two heights of figuring in the same quilt panel, one is the small beaded lines of the double needling, and then the larger inner diamond cells will rise slightly higher in effect. Again, disregard cell size and stitch spacing, even spacing needle to needle, as I want to put the double needles pretty close and do large diamond cells. On a men's jacket they'd be about 4-diamonds across, width-wise, in a size smedium'ish jacket, maybe 5-6 cells across on a larger size.
(really shitty leather quality/quilting example, disregard most of this too, just pay attention to general stitch type)