Example extracted from http://railsware.com/blog/2011/11/02/advanced-server-definitions-in-capistrano/

gistfile1.rb

server 'app1.reservoir.dogs', :app, {
  :user => 'nerd1',
  :ssh_options => {
    :keys => './keys/nerd.pem'
  }
}
 
server 'app2.reservoir.dogs', :app {
  :user => 'nerd2',
  :ssh_options => {
    :keys => './keys/nerd.pem'
  }
}