MultiAppHandler.rb

class MultiAppHandler
  def initialize(default_app, extra_app)
    @default_app = default_app
    @extra_app = extra_app
  end

  def call(env)
    request = Rack::Request.new(env)
    extra_app_routes = @extra_app.routes[request.request_method]

    if extra_app_routes.present? && extra_app_routes.any?{ |pattern, keys, conditions, block| pattern.match(request.path_info) }
      app = @extra_app
    else
      app = @default_app
    end
    app.call(env)
  end
end

original_app = Capybara.app
Capybara.app = MultiAppHandler.new(
  original_app,
  Rails2App
)