To access Split Mode you must press button 2 while you're in the main menu.
NMSVE's blue connect LED will flash indicating it is searching for a device to connect to. Once connected, the blue LED will go solid.
Split Mode is very similar to NMSVE's Default Mode. The octave slider and rotary knob retain their abilities from Default Mode, but what differs is which channels are triggered.
In Default Mode, you're only able to use the buttons within one channel at a time (chosen from the main menu). With Split Mode, the buttons are now "Split" between channel 1 & 2, allowing you to assign different instruments/effects to either channel via software.
As such, the first 6 buttons are tied to channel 1, while the last 6 buttons are tied to channel 2.