My grandmother made a lot of polyester doubleknit quilts that have survived to this day, so I say yes, you should be able to sew them together. I don't know if using a narrow zigzag would be better than a straight stitch or not. I know if you sew regular knits, it is better because it allows more give (for the stretch) and you get less popped stitches. I've never made anything with polyester doubleknit.