Another poster asked a very important question. Do you sew?
The two pieces of fabric would be joined together using a felled seam. This seam and all of the other resulting seams in your tarp should be “seam sealed” with a mixture of odorless mineral spirits and silicone or Permatex flowable windshield sealer (undiluted). The seam sealed portion, needle holes and all, will be the most waterproof portion of your tarp. Silnylon by itself is only waterproof up to 2 PSI or so. The seam sealant will make the material in the area of the seams stronger and more waterproof. I use a foam rubber disposable brush with edgeways strokes to apply the sealer.
Follow this link to Jay Ham’s article on how to sew a felled seam .
The folds of this seam impede water flow and the seam sealant reiforces the seam and seals up the needle holes. Seam sealing should be done under tension, that is, have the tarp set up and as taut or tight as possible. This will allow the sealant to enter and fill the needle holes. Make sure and seal the edges of the seams really well so water can’t get underneath.
Good luck with your tarp.
Party On,
Newton