Python character and word counts

I'm a beginner with python and would like to know how to use two txt files to count the characters as well as counter the 10 most common characters. also how to convert all characters in the file to lower case and eliminate all characters other than a-z

here's what i've tried and had no luck with:

from string import ascii_lowercase
from collections import Counter

with open ('document1.txt' , 'document2.txt') as f:
    print Counter(letter for line in f
                    for letter in line.lower()
                    if letter in ascii_lowercase)