I assume you use git bash and openssh.
Like what it’s written in the article, you can make a configuration file for ssh client that lists all of your accounts. You can write the following configuration in your own ssh client configuration file in ~/.ssh/config
Host account-one
HostName server.example.com
User user-one
IdentityFile ~/.ssh/key-one
Host account-two
HostName server.example.com
User user-two
IdentityFile ~/.ssh/key-two
What it says is you define two, kind of, “host aliases” named account-one and account-two. If you use them, when making connection, the ssh client will use the corresponding HostName, User, and IdentityFile for the server address, username, and ssh key file. With this you can use them to access your accounts and keys at even the same server.
In git, you can define two remotes using them
$ git remote add one account-one:repository.git
$ git remote add two account-two:repository.git
then you can push to those remotes
$ git push one master
$ git push two master