If you press “mixrack” on the buttons below the touchscreen there is a button which says talk back and you just assign where you want the signal from the talkback mic on the desk to go.Alternatively in editor there is a button for talkback assign

http://www.maltingsaudio.co.uk

other tools to bare in mind for comms applications:

You can send any input mic signal direct to an output socket or an ACE channel without assigning to a input channel. For example its already your talkback mic source just patch out onto the outputs going to the other mix rack. So you can patch this mic signal direct to the other console without blowing a mix or input channel.

Each mix has an external input if you wish to plumb a comm line to a mix permanently.
Obviously any source can go into the external PAFL input too. So you can talk direct to the engineer if he has ear buds in or is wedged.