'Each other' is always spelled as two separate words. Always.
It can be confusing to some because when pronounced, the words are said in quick succession. In other words, it sounds like it should be one word. Have a listen.
How can we remember that 'each other' is always two words?
My trick is to remember the definition. It’s about describing a relationship between TWO or more things.
Hope this helps. Good luck! 😃