1. Generate your ssh key:
ssh-keygen -t ed25519 -C "user@email.com"

2. Add your ssh keys in Gitlab under settings

3. Edit your ~/.ssh/config file with following content:

# User1 Account Identity
Host username1.gitlab.com
  Hostname gitlab.com
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/key_path.pub

# User2 Account Identity
Host username2.gitlab.com
  Hostname gitlab.com
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/key_path.pub

4. Test if you're able to connect to both accounts:

ssh -T git@username1.gitlab.com
Expected Output: welcome to gitlab, username1!

ssh -T git@username2.gitlab.com
Expected Output: welcome to gitlab, username2!