Config file settings
addkeystoagent yes
Host github.com
Hostname github.com
Identityfile ~/.ssh/defaultuser_private_key
Host github.com-username1
Hostname github.com
Identityfile ~/.ssh/username_1_private_key
Host github.com-username2
Hostname github.com
Identityfile ~/.ssh/username_2_private_key
The first Host section is if you want to have a default user setting. Can be removed if you want to explicitly set the username for individual git repositories.
Git remote settings for repo
git remote set-url origin git@github.com-username1:repo_owner_username/repo_name.git