If you look in the default key bindings, you can find the command that splits the screen. Then you'll need to write a plugin that uses sublime.Window.run_command() to execute the command you found through the key bindings file. http://www.sublimetext.com/docs/2/api_reference.html has details about the API. Taking a look at some existing plugins can always be of help to figure out how things work. The ones in the Default package tend to be fairly simple.