With “multi-gang” switch replacement (i.e. two, three, or four sliders on the Brilliant), there are some additional steps you’ll need to take when connecting the wires. Start by reading the section in the installation guide about replacing your switch, and note these additional instructions:
Installing 2-switch, 3-switch, and 4-switch ("multi-gang") Brilliants with more than one circuit
The 2G and 3G versions of the Brilliant Control are only certified to replace switches which control lights all on the same circuit.
With a 4G version of the Brilliant Control, you can control lights on two circuits, but it can only be divided down the middle. Switch 1 and switch 2 must be on one circuit and Switch 3 and switch 4 must be on the second circuit.
In this scenario, the neutral wire (usually white) associated with light switches 1 and 2 must be connected to the N terminal in between gangs 1 and 2. The neutral associated with lights 3 and 4 must be connected to the N terminal in between gangs 3 and 4.
Other wires: The rest of the line and load wires should be connected as normal. In most cases, all of the ground wires will be connected together. And the ground for the Brilliant Control should be tied into that bundle. If there are two separate ground bundles, tie the ground to the bundle associated with switch 1 and 2.