See above for the boost info (you trade extra amps for more volts).
As to the stacking thing...
"Stacked" batteries (two or more batteries end-to-end in series) add their voltages together. There's problems that can occur with the setup though and it's why many don't recommend it.
Basically voltage adds, but amp limits don't. Voltage is "oomph". Kinda like water pressure. (it's potential difference between poles...the more difference, the higher the voltage and the more the electrons are encouraged to move.)
So two 3.7 volt batteries each with a 4 amp limit (that limit varies depending on type) when stacked give a 7.4 volt power source with the same 4 amp limit. Now, If one battery in the set is mostly discharged and the other is fresh-charged, and you try to push all that charge through the imbalance, particularly for high loads, the imbalance can overheat one faster I guess. Or lets say you short the atty connector and the button gets stuck or you forget to lock the button, put it in a pocket/backpack/car storage box/whatever and the button gets pressed and the coil shorts. Any short will stress BOTH batteries quicker due to voltage being higher...and a short is 0 ohms or nearly so...so it's by definition over any battery's amp limit.
Basically even a single battery shorting is bad. And two in series is double bad.
The batteries can "vent violently" in some cases. That's why we say to use protected Li-Ion batteries. Other battery chemestries are safer...like IMR...but they can still vent. There's a thread around here with a guy who's IMR battery vented and expanded way out in his backpack.
So the "danger" with stacking batteries is that it's just a little bit easier to have a more serious venting issue with two batteries in the same mod. The extra voltage pressure helps to...exacerbate... the already bad-news short situation. Batteries should be "balanced" and checked. Etc.
This should give you the general idea. I'm not really an expert. But you can google "battery university" for more info. Here's a link:
Basic to Advanced Battery Information from Battery University
I should say I stack batteries in my box mods all the time. This is NOT a mech though. It has a regulator with over-current protection. And I use protected batteries each of which has over-current protection. So I have three layers of protection in series. And box mods I use can't build up pressure (the backs just slide off).