No, if you set both stages to 14 oz it'll take 28 oz of total pressure to break a shot. However, our brains tend to interpret it more like a single 14-16 oz break; the brain "forgets" the 14oz you're holding to get to the wall.
I'm sure you can find videos explaining this, so I'll only answer the other question you had about prevalence. Single stage triggers are more prevalent, but there are a decent proportion of people running a 2-stage, myself included. People have different preferences for different reasons, but the reasons I like a 2-stage:
- Taking up the first stage is how I "drop in" mentally for that shot; it basically begins my firing sequence, and I can always back off
- It makes an ND (negligent discharge, usually a stage or match disqualification) less likely, esp wearing gloves (note here: lots of folks will say "You just need better trigger control," and maybe they're right, I don't much care)
I've tried both, and discovered I prefer the 2-stage; I think for most people, you just need to try both for a while each and see what you like better. Because the trigger is one of the most critical interfaces you have with the rifle, I think it's worth buying twice and crying twice, if that's what it takes to get a fit that's right for you. You can always buy used and sell the one you don't want.
Good luck!