This is going to be easier if you take advantage of the Virtual Channels on the Apollo Twin. I'll show you how to do that first.
In Studio One, you'll want to change the Main output to the Virtual 1/2 pair:
Next, you'll want to Link Virtual 1 and 2 in Console:
Now you are ready to send live inputs and Studio One playback to the headphones, but first we need to tell the headphone jack to play the headphone mix:
To send playback from Studio One to the headphones, use the HP send fader. Your live inputs also have an HP send fader that you'll use to get the right levels into your performer's headset: