You're creating two JsonStore
s. This is like opening the same text file in two separate editors, then making changes and saving each copy of the file - only one copy will be saved, and the other will be overwritten. Try using the same store for each method.
jsonstore.py
from kivy.storage.jsonstore import JsonStore
import importedmodule as jb
JsonFileName = 'a.json'
store = JsonStore(JsonFileName)
def hello(s):
s.put('ten', v=int(10))
hello(store)
jb.hello(store)
importedmodule.py
def hello(s):
s.put('twenty', v=int(20))