apps.rb

# foo.rb
class Foo < ::Sinatra::Base

  class << self
    def dependencies; []; end
    def setup_application!; end
  end

  get '/' do
    'wubwub'
  end
end

# config/apps.rb

Padrino.mount("Main::App", :app_file => File.expand_path('../app/app.rb', __FILE__).to('/')
Padrino.mount("Foo",       :app_file => File.expand_path('../foo.rb',     __FILE__).to('/foo')