gistfile1.rb

class App1 < Sinatra::Base
  set :views, "/path/to/views"
  set :public, "/path/to/static/files"

  def strong(text)
    "<strong>#{text}</strong>"
  end

  get '/' do
    strong "Hello World"
  end
end

class App2 < Sinatra::Base
  set :views, "/path/to/more/views"
  set :public, "/path/to/more/static/files"

  def em(text)
    "<em>#{text}</em>"
  end

  get '/' do
    em "Hello World"
  end
end

class Middleware < Sinatra::Base
  get '/foo/bar' do
    @app.call(env)
  end
end

gistfile2.rb

use Sinatra::Base do
  get '/' do
    ... do cool shit ...
    @app.call
  end
end