This gits is explaning about how to use mulitple SSH keys in multiple github accounts!
I found some articles but they are very confusing. So I think I should just summary this up so that I can remember it.
I am using MacOS by the way. I am using these github1-username and github2-username as sample accounts. Please replace them to proper account :)
$ ssh-keygen -t rsa -b 4096 -C "[email protected]"