Struggling with Groups (Associations) in Z-Wave JS UI

I have two separate Fibaro FGS-223 devices. Let’s call them DevA and DevB.

The second button on DevA (S2) is controlling some outdoor lights.

The output Q2 on DevB is not connected to any device, though I do have a toggle switch connected to S2.

Here’s what I want to do: whenever S2 on DevB changes its state, I want to turn on Q2 on DevA.
Ideally I also want the other way around, so that both logical switches that correspond to S2 in both devices are always in sync.

I suppose I must create a new association on DevB with:

  • Node Endpoint: Endpoint 2
  • Group: On/Off(S2)
  • Target Node: DevA
  • Target Endpoint: Endpoint 2

I also tried some other combinations, but I could not get it to work (no matter what, S2 on DevA does not turn on when I turn on S2 on DevB).

FYI, both devices are added to the network with S0_Legacy security.

What am I missing here?