With the great help from @gd35 and AI I finally succeeded in pairing an Ikea Bilresa Dual Switch on the OTBR and also pair it to the Matter Binding in OH:
Only HW needed was a re-flashed Conbee II/III USB Dongle …
Only SW used was OTBR CLI tools + OH. No phone App.
The next one will go a lot faster …
It was quite a journey … 