With the split view window active, put the mouse at the top of the screen to make the toolbar show and click on the green button again on one of the apps. That will remove that app from split view. You will have to select the other app also and do the same to remove it from full view.
You can also use Control-Command-F keys to remove the active app in Split view, then select the other app and use the same keys to remove it from Full View.
You can also go to Mission Control window (my keyboard is set to either the F3 key or Control-Up Arrow keys. From there, you will see the split view app combined. You can put the mouse over the split view icon and arrow keys will appear at the upper left of the icon. Click it and it will remove the split view apps.
