First create a file in your default Star Wars Galaxies folder (IE: C:\Program Files\StarWarsGalaxies) called "user.cfg".
Then edit the user.cfg file with notepad. Add the following lines:
Save the files and close.
Make sure that you set the file to read-only.
Now run your game like normal, and load into the game world. Once your first charecter is loaded into the world, load SWG again the same way, only login with your other account. The game should load like normal, and depending on your system might play a little slower, but now you have both charecters in!
When in game press the alt + tab key to switch between game clients and your good to go.