SSH config - same host but different keys and usernames

Yes, only the section headers (the Host or Match lines) are searched for – everything else is only applied as a setting. In other words, if you connect to foo@bar.com, OpenSSH will only look for a section titled Host bar.com.

So if you have Host github_username2, you must use this exact same "hostname" in your Git remotes as well. OpenSSH will not find this section if you use git@github.com.

However, that is not what causes authentication failures. When connecting to GitHub via SSH, you must use git as your username – the server will recognize you based on the key alone. (In other words, the "git@" in "git@github.com" is actually the SSH username that GitHub uses – not some kind of URI scheme.)

So a correct SSH configuration would be:

Host github_username1
    Hostname github.com
    User git
    IdentityFile ~/.ssh/rsa_1
    IdentitiesOnly yes

Host github_username2
    Hostname github.com
    User git
    IdentityFile ~/.ssh/rsa_2
    IdentitiesOnly yes

with this Git configuration:

[remote "origin"]
    url = github_username1:username2/repo.git

(It doesn't matter where you specify the SSH username – you can have git@ in the URL, or you can have User git in .ssh/config, or both.)

Alternative Git configuration to automatically switch accounts depending on repo path:

1. Create a file ~/.config/git/config.user1 containing:

   [url "github_username1:"]
       insteadOf = git@github.com:
   

2. Create a config.user2 file that's the same except with "github_username2".

3. In your main ~/.config/git/config file, tell Git to "include" one of the two files depending on which directory you're at:

   [includeIf "gitdir:~/projects/"]
       path = ~/.config/git/config.user1
   
   [includeIf "gitdir:~/src/work/"]
       path = ~/.config/git/config.user2
   

4. Now, whenever you're at ~/src/work/, cloning anything from git@github.com:[etc] will automatically rewrite the URL to github_username2:[etc].