You don’t.
The IDR IS the mixer, you only have one IDR so you only have one mixer.
The T112 and I guess T80 are control surfaces, not mixers. Think of them as big expensive remote controls for the IDR, they are not mixers and without an IDR (each) they do nothing.
There is no way to attach 2 control surfaces to a single IDR MixRack.
If you wish to use what you have as a FOH/Monitor configuration you’ll need another IDR and a network card (ACE is the cheapest option although Dante gives more flexibility and functionality) in slot B of both IDRs.
The onboard ACE socket goes to the surface, the slot B interface connects the 2 MixRacks.
The good news is you don’t need another IDR the same size if you use a digital split in this way, an IDR16 will suffice.
This document shows connections using ACE.
https://www.allen-heath.com/media/Understanding-Ace-rev-18dec12.pdf
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