Thank you all for your quick and clear explanations. I get it and will use to it. I was a little confused, because if I remembered well, that on my former keyboard each button touched got 'on' and the other got 'off'. To have both 'on' you could press' both' knobs 'on' or 'off'. Thought it was something that I could setup.
Now I know how to handle it. For me it is already difficult to touch the right black and white keys, now I have to learn to touch the rights buttons too
Grtz. Ton
@jwyvern: tried as you described and it works for me, simple but effective suggestion, which I have to practise to get it 'automatic' done