There are a few options you can take here. In all options you would store the other key in its own file, /home/user/.ssh/id_rsa for these examples.
1) When you want to use a key other than ~/.ssh/id_rsa specify it with the -i argument
ssh -i ~/.ssh/id_rsa.otherkey firstname.lastname@example.org
2) If you want to use the key multiple times in your current session, add it to your ssh-agent with ssh-add:
3) If you want to set this up more permanently you can specify the keys in ~/.ssh/config along the lines of: