Setting up a split menu
Step 1
First thing you will need to do is update you menu to have child menu items. This is done in the relevant menu via Joomla's Menu Manager. An example of a menu with child menu items can seen below:

Step 2
The next thing you need to do configure your top menu settings to accommodate the Split Menu. See the example top menu module (published to the "user3" position) settings below. You will notice that we have set the "End Level" to "2". The reason for this is so that Joomla knows to only display 2 levels of menu items before splitting contents into the sidebar menu.

Step 3
Now move on to your sidebar menu. If you don't have a menu module published to your sidebar, simply duplicate the existing top menu module and publish it to the splitleft or splitright module positions.
Once you have the module published to your chosen position, you can proceed to adjusting the modules settings. Notice how the "Start Level" is set to "1". This is important, as it tells Joomla that the menu should only load menu items from second level of menu items.

Example of Split Menu in action
The below examples are of how your menu will look once it's setup. Please note that the actual styling of these will vary from themelet to themelet.
Spit menu with sub items displayed in the topleft module position:


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