Hi. The problem in my case is that the device which I need to control via the right paddle of this switch is the 2nd channel of a Fibaro double relay. So I could have done it via direct association without involving OpenHAB at all, but these switches do not support multi-instance associations, so I cannot add the 2nd channel of Fibaro relay to group 2 or 3. That’s why I need OpenHAB here.
And I could do the trick in Openhab 1, because I could associate one item with the switch_binary/switch_dimmer command class (which is sent by the switch to devices in group 1), and the other item with basic command class (which is sent by the switch to devices in groups 2 & 3).
But I can’t do it now any more in Openhab 2/3/4.
The Z-Wave controller doesn’t know the association group the message comes from, but it could map the switch binary command class to openHAB channel #1 and the basic command class to openHAB channel #2. Whether the Z-Wave binding does supports this, I don’t know either, but
must have some meaning. 
I really don’t know, but you could give it a try (say dimmer on one and switch on the other) and @roher could test. I think the “separate” has to do with this in the XML.
<!-- CHANNEL DEFINITIONS -->
<channels>
<channel id="switch_binary" typeId="switch_binary">
<label>Switch</label>
<properties>
<property name="binding:*:OnOffType">COMMAND_CLASS_SWITCH_BINARY,COMMAND_CLASS_BASIC</property>
</properties>
</channel>
</channels>
What a number of my switches do is send the Basic set to the controller when they are operated manually, but OH sends the switch binary command when operated with Zwave. Without the Basic tied to the channel OH will not update the status of my devices when operated manually. Don’t know about this device. It sounds like it sends switch_binary on the right (even when operated manually) and basic on the left.
Did some thinking on this and I’m not sure I understand are you talking about Multi-Channel Association? At least on the UI I get options on my devices to associate device endpoints. I do vaguely recall problems with Associating endpoint 2 between dual switches, but did get endpoint 1 to react (don’t remember any details anymore). That leads me to; can you switch the wires on the Fibaro in the box so the item you want to control is on the first EP and then just associate the device? Sometimes EP0 and EP1 are mirrors. You could test first with the current item on Ep1 before messing with the wires.
I cannot do it, because in my setup I need both relay endpoints to be associated with my TKB dimmer:
EP 1 (bathroom light) - single click of the right paddle - group 2.
EP 2 (bathroom fan) - double click of the right paddle - group 3.
So now I can control bathroom fan only via my smartphone or via the rules (by the humidity sensor for example). With Openhab 1 I could control it physically by putting the controller into group 3 and linking the item to BASIC command class.