Duct tape or use for smaller cover ups. If you sew it, it will only create a perforation that will make it easier to tear in the future. Tarp isn't that expensive. The only other idea would be to put the 2 pieces together and fuse them with an Iron. You put the 2 pieces together and use brown paper bags/shipping wrap under the tarp to protect the surface you're working on and on top of the tarp between your iron and the brown paper bag/shipping wrap. Worth a shot.