On my double I have in the past just used regular laced reins on the snaffle and some 1/2" smooth curb reins I got off ebay. I recently got some calfskin plain reins with stops on ebay that I use as my snaffle reins now.
I hate my curb rein too - it's the standard 3/8 smooth leather rein that came with the bridle. I had not thought of getting a slightly wider rein for that. For my snaffle I have very thin biothane reins -- the ones with the little pimples on them. I love them. But I also use 'eventer' rubber reins exclusively on all my snaffle bridles. I wish you could buy bridles without the reins - so many come with laced or web reins and I never use them and always end up almost giving them away -- brand new and unused -- at local used tack sales.
I have the albion suede lined rein for the snaffle rein, in oversize, so it's really long, and had my saddle fitter/bridle maker add stops the width of my hand. It's a 5/8 inch rein. My curb is the matching half inch oversized curb rein, plain flat leather. I love this setup.
Before that I had Jerry's make me reins, with the snaffle rein having stops the width of my hand. But I prefer this, with the snaffle rein slightly thicker and with a little more grip.
I do have a friend who will put sticky wax on her snaffle rein before a test. For me that is too sticky.