I tried this and it worked a while. But then somehow my printer pdf24_a was the same as pdf24_b and pdf24_b was not listed anymore but I cannot create a new pdf24_b because the name already exists. Slightly confusing.
What I did is call pdf24 and go to settings. In section "pdf printer" there is a red (+)-sign where you simply can add a pdf24-printer and give a name.
That's it. Just wanted to share.