To use Splitting functionality, first we need to allow splitting the PoS order at the PoS session settings:
This checkbox will enable Splitting control button at the main PoS UI:

Click on the Split button and select the lines that we need to split to a new PoS order from the current one and click the Split button:
The order will get split into 2 separate orders like below:
The orders will appear with a deactivated order button.
At the Order monitor the lines will be displayed as two separate Orders: