Thanks for the hint JPV! Reminds me that I should take time to read the MorphOS Library more thorougly...

I followed the guide to set the 1st adapter to Port 0 - Merge with USB, Port 1 - Don't touch and the 2nd adapter to Port 0 - Don't touch, Port 1 - Merge with USB. Next I set up the port numbers in the Action Window accordingly. At my first attemp I failed, because I did not realize I had to set the port numbers for X and Y axis separately. But now it works as supposed with 2 ports.