Controller issue can't bind last 8 skills on hotbar

I'm playing poe 1 on windows using a newer xbox controller. I can bind skills to abxy and abxy plus right trigger, and abxy plus left trigger by pushing in the right thumbstick and hitting the appropriate trigger.

But when I try to press both triggers in order to bind the skills for when both triggers are pressed it doesn't let me and the right thumb stick button greys out when I have both triggers pressed.

What am I doing wrong, and how do I bind those last 8 skill slots using both triggers depressed?