Hi Kelly,
You do not have to pay for both, when you get Das Trader Pro with IB or any other broker. It comes with a demo account as well. That is included in your monthly fee, you will only be paying $150.
It is also very easy to switch from Live to Demo with Das Trader Pro. There is one login only but once you log in you have several options to switch from Real and Sim, you can do it manually on the montage, you can define it by hotkeys or you can change the overall global setting to Real or Sim.
This post has more information on how to switch between sim and real. Hope this helps. Thanks.