I recently ran into a situation where I needed to be able to access both github and gitlabs on my laptop with different accounts. I had no clue where to begin but I found this article https://code.tutsplus.com/tutorials/quick-tip-how-to-work-with-github-and-multiple-accounts--net-22574 which walked me through the setup process. My config file was slightly different than what was specified in the article:
#Default GitHub Host github.com HostName github.com User git IdentityFile ~/.ssh/id_rsa #Gitlabs for Work Host git.WORK_TLD.ca HostName git.uwaterloo.ca User git IdentityFile ~/.ssh/id_rsa_WORK
Everything was pretty straightforward after that.