Lining your potholder can be very effective and poses an extra safety precaution as you will be handling hot objects. Though it will be easier to crochet two or duplicates of your current piece and layering it for a double-thick design, seaming two together might make it bulky, and it would be a more extended crochet project. Another method you can try is lining them with one or two layers of cotton fabric. It's a more manageable and faster project; you can either hand-sew or use a sewing machine. If you would like more details and step-by-step instructions on accomplishing this, read this article on How To Line Potholders With Fabric.