It's quite normal to have different private keys for different git servers, but how do you provide a different identity file like you do with SSH?
The answer comes in a text file;
Create a text file in your ssh directory, which is usually
$ touch ~/.ssh/config
Then open it with your favourite text editor, for me, this is currently atom:
$ atom ~/.ssh/config
Now, we can use this file to configure different hosts. The following should be pretty self explanatory. For your information, in case you're a bit confused, the identity file is your private key.
Host example.com HostName git.example.com User git IdentityFile /Users/adam/.ssh/yourkey IdentitiesOnly yes
You'll notice the host and hostname are different. This means you can have a host configured for example.com which actually points to another hostname, such as a source control server, e.g. git.example.com.