config.ru

require 'sinatra/base'

class A < Sinatra::Base
  get '/this' do
    'this from A'
  end
end

class B < Sinatra::Base
  get '/that' do
    'that from B'
  end
end

class Rack::Chain
  def initialize(*apps)
    @apps = apps
  end

  def call(env)
    @apps.each do |app|
      status, headers, body = app.call(env)
      return [status, headers, body] unless status == 404
    end
    [404, {}, ['No app matched']]
  end
end

run Rack::Chain.new(A, B)