Because you cannot use the same SSH key for multiple accounts on Github, Gitlab, Bitbucket, etc, you will need to generate multiple public/private keys and use them accordingly.
Let's create a ssh key
ssh-keygen -t rsa -C "firstname.lastname@example.org"
You will be prompted for an optional password. It depends on you if you want to have a password or not for this.
Copy the public key (Mac OS)
pbcopy < ~/.ssh/id_rsa.pub
And paste it in your git server account SSH keys page. You may see here how
If you use only one account, then you can stop here.
Generate a new SSH key with a different name
ssh-keygen -t rsa -f ~/.ssh/second-account -C "email@example.com"
Now, you need to create the file
~/.ssh/config with the following content:
Host bitbucket.org User git Hostname bitbucket.org PreferredAuthentications publickey IdentityFile ~/.ssh/id_rsa Host bitbucket-second User git Hostname bitbucket.org PreferredAuthentications publickey IdentitiesOnly yes IdentityFile ~/.ssh/second-account
Now, to use the primary ssh key to clone a project you use the following command
git clone firstname.lastname@example.org:username/project-name.git
And for the second account you'll have to change the hostname in the url like this:
git clone git@bitbucket-second:username/project-name.git
The example was provided with an example to bitbucket, but in the same way it works for Gitlab, Github and any other git servers.