HH-66 won't work on poly tarps to the best of my knowledge - if it does I apologize, but it's a very specific adhesive designed for vinyl. Polyethylene is very adhesive repellent. The best way to put a seam in it is either sewing or some other mechanical means, or else heat seaming with an iron (quite easy in my experience, but YMMV).
If you're going to go grommets - might I suggest spray gluing a strip along the edge, or folding the edge over and ironing first to make sure there is enough meat for the grommet to hang on.
If it were me I'd strongly consider actually doing a circus seam (this is actually a sewing term - I can't picture what CO was suggesting either, though it sounds like he knows his stuff), but instead of sewing, punching grommets through both tarps after you fold the edges together. No messing with bolts or zap straps or bungee balls (moop).
If it were me, and I needed a strong seam in a poly tarp that's likely what I'd do. Something like this, only with grommets, instead of sewing:
(Image is not mine - Stolen from a backpacking site)
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