Howto use multiple SSH keys for password less login

http://www.cyberciti.biz/tips/linux-multiple-ssh-key-based-authentication.html 

Step # 1: Generate first ssh key

Type the following command to generate your first public and private key on a local workstation. Next provide the required input or accept the defaults. Please do not change the filename and directory location.
workstation#1 $ ssh-keygen -t rsa
Finally, copy your public key to your remote server using scp
workstation#1 $ scp ~/.ssh/id_rsa.pub user@remote.server.com:.ssh/authorized_keys

Step # 2: Generate next/multiple ssh key

a) Login to 2nd workstation

b) Download original the authorized_keys file from remote server using scp:
workstation#2 $ scp user@remote.server.com:.ssh/authorized_keys ~/.ssh

c) Now create the new pub/private key:
workstation#2 $ ssh-keygen -t rsa

d) Now you have new public key. APPEND this key to the downloaded authorized_keys file using cat command:
workstation#2 $ cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys

e) Finally upload authorized_keys to remote server again:
workstation#2 $ scp ~/.ssh/authorized_keys user@remote.server.com:.ssh/

You can repeat step #2 for each user or workstations for remote server.

Step #3: Test your setup

Now try to login from Workstation #1, #2 and so on to remote server. You should not be asked for a password:
workstation#1 $ ssh user@remote.server.com
workstation#2 $ ssh user@remote.server.com