hmm I can see why you arent sure about the water tightness, in fact actually that was what I was wondering.
but perhaps I know of a method in which it would be watertight
it is also used allot in the seams of the more expensive tents etc, it is actually exactly the same as with your seam but the "Sandwich" is made longer then folded so that three layers will touch each other sort of like a cinnamon roll and then its stitched.
I will draw up what I mean and post it in a few..... and here we go
But that wont be necessary unless it leaks, so let me know how the tarp/sandwich performs under water.
I'm very curious how it will perform
Yours sincerely Ruud