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 [email protected]
2) If you want to use the key multiple times in your current session, add it to your ssh-agent with ssh-add: ssh-add ~/.ssh/id_rsa.otherkey
3) If you want to set this up more permanently you can specify the keys in ~/.ssh/config along the lines of:
Host shortcut
HostName server.example.com
User user
IdentityFile /home/user/.ssh/id_rsa.otherkey