On one mating edge of one tarp (a), place grommets every 12".
On matching edge of other tarp (b), fold back 9" to create a double flap. Place grommets matching the 12" placement on other tarp.
Take 1/4" rope, cut into 30-32" long pieces. Tie into a loop running 12.5" from loop end to knot.
Match grommets on tarp a to grommets to tarp b with tarp a end between the two flaps of tarp b. Starting at one end, feed a rope loop thru grommets in b/a/b order. The second rope goes thru wnd grommet set same direction, then thru the loop of the previous rope. Follow that pattern to the end and tie the last with a stopper knot.. It creates a waterproof seam, while keeping the convenience of 2 smaller tarps.
I'll see if I have photos on the home computer.
M*A*S*H 4207th: An army of fun.
Fire Spinners- 2B1, ask1. Oregon Fire Conclave
Yes, I am the arbiter of doing it right or wrong. Guess which one you are!