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.