Because we are all adults who can make up our own minds.
We can give you all the advice, and recommendations in the world, but you makes your choices.
I am not a stacker with my mech mods, but I do not use protected, yet I would advise protected batteries in a wood mod, and also point out that with wood mods, a fair amount of time it is probably best not to have an atty with less than 1.8ohm resistance on it.
If you do choose to stack, ensure that both batteries used are of the same brand, and both have the exact same charge in them, otherwise, the battery with more charge will be trying to power the device and the other battery, which is the equivalent of a British seventies public information add where someone shouts, 'Jimmmmmyyyy!', as a child is fried by the national grid. Well, not really, but it isn't great.