To add multiple ssh keys to your system, you need to edit config file ~/.ssh/config and put lines like below, this will also help store password for your ssh in keychain and use agent forwarding so you can use your keys on virtual machines like vagrant
Host *
UseKeychain yes
AddKeysToAgent yes
ForwardAgent yes
IdentityFile ~/.ssh/id_rsa
IdentityFile ~/.ssh/marbl
more on that here