Is the creative shelter tarp's seam that noticable?
I have the Creative Shelters tarp, too. Yes, it is noticeable but mostly to people who are looking for it.
For instance, I notice it but I've never had anyone else mention it at any showing.
It's about a one-inch wide horizontal seam where the two pieces of vinyl are joined together. This one-inch double-ply section does reflect a little more light. If you look at it from the rear that strip is a little darker (the exact and expected opposite). I doubt this phenomenon is unique to the Creative Shelters tarp. The only way around it is likely a one-piece tarp (hard to find in 10x20 size) or billboard vinyl.
I have to agree that layering (unless you're gluing the layers tightly and evenly together) will likely reduce the contrast due to random reflection angles and minute changes in focal depth. I say leave it alone!