The dxNothing window has nothing to do with opening more than one client. dxNothing is a blank window for showing video FX, which will let you SEE the other windows once they are up.

What lets you open more than one client is the Inner Space and ISBoxer updates. If you do not have one or the other, you will not be able to launch more than one client.

Here's the important part...
There is both an ISBoxer update (39.1.1126.1) and an Inner Space update (5477, development build of Inner Space). You will need them both. Neither are currently pushed updates, meaning that you will need to specifically do the following:
* ISBoxer: go to the Help menu, and select About ISBoxer Toolkit. If your version is earlier than 39.1.1126.1, it should have a button for you to click to update.
* Inner Space: right click IS and select Patcher, check "Download development (test) patches" then close and re-start Inner Space to let it patch
Again, if you do not have Inner Space build 5477 or later, AND ISBoxer 39.1.1126.1 or later, you should expect a message from the SWTOR launcher saying the game is already running.