little_proxy.rb

# Run the little Sinatra app on port 3000 and the big Rails app on
# port 3001.  Run this file and hit them both at port 4000.

require 'rubygems'
require 'rack/proxy'
require 'thin'

class LittleProxy < Rack::Proxy
  def rewrite_env(env)
    if env['PATH_INFO'].match(/^\/little\//)
      env['SERVER_PORT'] = '3000'
    else
      env['SERVER_PORT'] = '3001'
    end
    env
  end
end

Rack::Handler::Thin.run(LittleProxy.new, :Port => 4000)