This option will allow you to open up a new "split screen" type view which is separate from GSAK. It is actually another program (like the macro editor) and communicates with GSAK via a client/server paradigm. This allows you to move the screen to another monitor and position or maximize to suit. You can use the "Dual screen" as a substitute for the split screen or in addition to.
Notes about the "Dual screen"
1. You can have both a split screen and dual screen running at the same time
2. You can have a different format in your dual screen to your split screen
3. Uncheck the "make active" (recommended for dual monitor users) box to let GSAK take focus when the dual screen is started. For single monitor users you may want to leave this box checked otherwise it may appear to you that the dual screen is not showing on start up.
4. The dual screen program will automatically be shut down when gsak terminates.
How would a single monitor user take advantage of this feature?
Perhaps there is a format you like to look at every now and again but don't want to make it your permanent split screen format.
By setting this format to your "dual screen" you can then just click on the Dual screen program in the task bar to see. Then back again to GSAK when finished. I would think this may be a better alternative than having to continually swap formats in the split screen.