In my existing 18i20 set up, I have analog inputs 3+4 and 5+6 hooked up to receive my synths and hardware effects. I also have a line mixer and patchbay which help route audio to these stereo pairs in whichever configuration I need.
To allow Push to access these analog inputs as well, I went to Focusrite Control and manually assigned them to 18i20 ADAT outs 1+2 and 3+4 respectively. They then appear as inputs 9+10 and 11+12 on the Push.
It was a bit trickier setting up an effects loop to send audio from the Push to my hardware effects chain and back via ADAT. I needed to create a custom “headphone” submix and use 18i20 outputs 9+10, which are shared with the second headphone port, because the direct monitoring options for incoming ADAT audio are limited. The custom submix has everything turned down to zero except whatever’s coming in on ADAT inputs 1+2, which correspond with outputs 9+10 on the Push.
So the audio goes out of the Push via 9+10 over ADAT, which the 18i20 sends to its analog outs 9+10, through my hardware effects chain, then back in on 18i20 analog inputs 3+4 as described above. All the 18i20 settings are stored on the hardware so a computer isn’t needed after initial setup.
Hope this is helpful.