Gitlab won't allow reuse of a public ssh key for multiple accounts. To get around this you need to create a second ssh key for the second account.

Create or modify your `~/.ssh/config` file:

```
# normal                                                                                                                                                                  
Host gitlab.com-work_username
     HostName gitlab.com
     PreferredAuthentications publickey
     IdentityFile ~/.ssh/id_rsa

# second                                                                                                                                                                  
Host gitlab.com-personal_username
     HostName gitlab.com
     PreferredAuthentications publickey
     IdentityFile ~/.ssh/second_key
```

Check that both authenticate correctly with Gitlab:

```
ssh -T git@gitlab.com-work_username
ssh -T git@gitlab.com-personal_username
```

Then in the repo you want to use the second account for, edit the `.git/config` file:

```
[remote "origin"]
        url = git@gitlab.com-personal_username:group-name/repo-name.git
```

Added the `-personal_username` after `gitlab.com`

---

To clone the repo initially you can use:

```
GIT_SSH_COMMAND='ssh -i ~/.ssh/<second_private_key> -o IdentitiesOnly=yes' git clone <repo_url.git>
```