what is wrong with my sub / slave config

[Twan]

Hi.

I have two physical zwave switches A and B. I can switch them both independently just fine.
Now I want the other one to switch at the same time, so I made B a sub/slave of A.

On the dashboard, when I press the A switch, I see B lighting up as well. But in real live the switch does not come on. The other way around as well, when both are on, and I turn off A, on the dashboard, B goes down, but in real live the lamp connected to B stays on.

What I do wrong here?

grtz,
Twan