"Inner" but not "outter"?
Solution 1:
Many English words have a double consonant when following a short vowel. Consider "biter" and "bitter": the double-t signals that the vowel should be the short i vowel. Thus, you write "inner".
For "outer", "out" is already a two-letter vowel, so it doesn't change sound when you add on other letters at the end, hence "outer".