Polyester is washable. No worries there. You of course want to prewash all the fabrics before starting. I don't know the weight of the polyester fabric, but even if they are different weights, it doesn't mean you can't sew them together. Afterall, the fabrics will be on opposite sides of the comforter. You will naturally have to adjust your machine tension, but other than that I don't think it would not work. One hint with the cotton fabric, they will shrink, so give them a good wash on HOT. And...it might bleed as well, so you might want to make sure its the only thing in the load. Good Luck!