Sometimes you need more accounts than one for access to Github or Gitlab and similar tools. For example you can have one account for your projects at home and second account for your company.

Case 1: Multiple accounts on Github

Create SSH keys with different names

$ ssh-keygen -t rsa -C "your_name@home_email.com"

When you see this message

Generating public/private rsa key pair. 
Enter file in which to save the key (/home/user_name/.ssh/id_rsa):

Enter unique name, for example:

id_rsa_home

Next, you'll be asked to enter a passphrase.

So, you'll have created SSH key for your home account, now you can generate SSH key for your company account.

Call SSH key generator again with second mail.

$ ssh-keygen -t rsa -C "your_name@company_email.com"

Enter name for file

id_rsa_company

After all steps you can check that all keys were created.

$ ls ~/.ssh

You should see a similar files list:

id_rsa_home  id_rsa_company  id_rsa_home.pub  id_rsa_company.pub

Now you need a config file for organise these keys.

$ cd ~/.ssh/
$ touch config
$ nano config

Add into config file:

# Home account
Host home.github.com
  HostName github.com
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/id_rsa_home

# Company account
Host company.github.com
  HostName github.com
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/id_rsa_company

Next you'll delete cached keys

$ ssh-add -D

If you see a message

Could not open a connection to your authentication agent.

Then enter:

eval `ssh-agent -s`

and try again previous command.

Next, you can check that your keys were added:

$ ssh-add -l
2048 d4:e0:39:e1:bf:6f:e3:26:14:6b:26:73:4e:b4:53:83 /home/user/.ssh/id_rsa_home (RSA)
2048 7a:32:06:3f:3d:6c:f4:a1:d4:65:13:64:a4:ed:1d:63 /home/mateusz/.ssh/id_rsa_company (RSA)

If you haven't any entries then you should add your keys

ssh-add ~/.ssh/id_rsa_company
ssh-add ~/.ssh/id_rsa_home

Now you can check connection

$ ssh -T git@home.github.com
Hi home_user! You've successfully authenticated, but GitHub does not provide shell access.

$ ssh -T git@work.github.com
Hi company_user! You've successfully authenticated, but GitHub does not provide shell access.

Note! Check the last paragraph of this tip.

Case 2: Account on Github and Gitlab

This is very similar case to the previous. I won't describe it step by step, because all steps are the same. I'll add only example config file.

For example you have own account for home works and company account on gitlab.

# GITLAB
Host gitlab.company_url.com
   HostName gitlab.company_url.com
   PreferredAuthentications publickey
   IdentityFile ~/.ssh/id_rsa_company

# GITHUB
Host github.com
   HostName github.com
   PreferredAuthentications publickey
   IdentityFile ~/.ssh/id_rsa_home

The test connections

$ ssh -T git@gitlab.company_url.com
Welcome to GitLab, CompanyUser!

$ ssh -T git@github.com
Hi home_user! You've successfully authenticated, but GitHub does not provide shell access.

As you probably have seen, prefix on hostname isn't required.

You may need to set git config user details for any project.

It's required to distinguish your accounts.

$ cd ~/home_project
$ git config user.name "home_user"
$ git config user.email "your_name@home_email.com" 

參考文章：Multiple SSH keys for different accounts on Github or Gitlab