Why did you write two different stories in one novel?
The short answer is: I haven't
The long answer:
So, my original plot for the story was the concept that behind the scenes of all the Xiaxia universes someone or something has to run around cleaning up after all the usual character types; that spend all their time going through precious items by the truckload to level up while leaving devastation in their wake.
The story follows a young boy who discovers he's one of the people who is responsible for fixing the mess.
The young female Cultivator who he meets (and possibly becomes a Dao Partner - not spoiling it) was intended as a character to introduce the rest of the Cultivation world to the readers. I realised quickly, that it would take a long time for the boy to leave the mountain and due to the nature of that mountain he would have no real reason to leave - apart for things he can't make himself.
So readers would get stuck there with him without seeing what was going on outside the Mountain or any impacts he has on the greater world for ages - which as an author trying to explain things to readers was not useful.
My initial drafts (you can see them on the WW forums if you really want to) had a three year time skip after the boy was left on the Mountain by the old man. He went from being a geese herder to a badass in one chapter with a few comments about the Mountain basically dumping everything he ever needed to know into his brain after three years of training.
Basically a plot hole. Boring and predictable.
Thus I decided, I wanted the boy to meet a girl - that was herself an MC as much as he was - and have her be the reason that the boy eventually interacts with the Cultivation world.
The problem at least in my mind, was that I did not want a stereotype beautiful, fairy, goddess to just be dropped in on him with no lead up or motivation - other than to react to him as the MC. I find that I do not enjoy novels where every other character is defined in terms of their connection to the MC - if they do not interact with the MC or the MC doesn't know about them, they do not exist.
Now, in XiaXia novels female MC's and Male MC's have to basically be perfect humans. The Cultivation is always given as the reason why and then the novels get on with the hanky panky or the fights.
But my brain kept asking questions. Like:
Why would a goddess like character even notice a peasant boy with no Cultivation?
How would a peasant boy meet a goddess type character?
How come she's a goddess?
What's a goddess doing where a peasant can meet her anyway?
So I started fleshing out the motivations behind the female character as well as why the male one would match her as a counterpart despite starting from zero.
In the process, my plot has become a 'two worlds collide' sort of story. With the boy being rebuilt through his adventures on the Mountain and forces conspiring to tear the Goddess out of her comfortable place into the boy's world - thus pulling him into hers and visa versa.
As I'm writing this at Chapter 67, the two worlds are basically not in contact with each other yet that anyone can see; attentive readers will have spotted however, that the boy's abilities have already started to affect the female's circumstances - despite their never having met and living three days travel (and a world) away from each other.
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The next paragraph is a semi spoiler - if you haven't read the chapters yet, feel free to skip
In fact, it is because she's having strange good/bad luck to discover all the plotting happening that will accelerate the mess and ultimately lead to her being on the Mountain; whereas without the MC, she would have got married off at fifteen (Ancient Chinese Adulthood for girls - look it up), her Second Uncle would take over the Clan, the Sect would have been crushed. Basically her story would have been cut short. And Golden Crow Planet would end up eventually dead.
For anyone wondering the Enforcer Sect is important. So are her Female Cousins.
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This is also why they are both so young. I needed them to be within a year of each other age-wise but because she couldn't be already engaged. I had to start the start the story with the female MC just over fourteen; by fifteen she would have been married off by the Clan as an adult, regardless of her father's wishes.
However, the female has her own reasons, viewpoints, thoughts, friends and desires just like the boy does. I understand it may alienate some readers - and as a writer it's driving me batty writing all this character development - but I can't go any faster than I am in building the characters up without - just basically writing chapters that would basically summarise as going 'And the boy became awesome and the girl was hot - they fell in love and killed all their enemies, the end.' (Not necessarily the ending of the novel)
So what was initially meant to be a chase-fight scene/a few chapters where the girl is fighting and losing on the mountain to be saved by the boy - got hit with the whys - and I had to write her properly.
The two major plot lines will merge eventually; I promise
PERTIMBANGAN PENCIPTA 
WheeledWriter
WheeledWriter I hope this helps answer some of the most prevalent questions I'm seeing in the comments.
Let me know if there's anything else that needs answering
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