ssh-keygen -t rsa -C 'myemail@example.com' -f id_rsa_mygit

# company github account
Host github.com
   HostName github.com
   IdentityFile ~/.ssh/id_rsa
   IdentitiesOnly yes

# my github account
Host github-mygit
   HostName github.com
   IdentityFile ~/.ssh/id_rsa_mygit
   IdentitiesOnly yes

ssh-add -D && ssh-add -L # reset all ssh keys, optional
# add them into ssh-agent
ssh-add ~/.ssh/id_rsa
ssh-add ~/.ssh/id_rsa_mygit
# ...other keys if you want
# test connection
ssh -T git@github.com
ssh -T git@github-mygit

git clone git@github-mygit:xxxxx/xxxxx.git

...
[remote "origin"]
    url = git@github.com-mygit:xxxxx/xxxxx.git
    fetch = +refs/heads/*:refs/remotes/origin/*
...