Hi,
I’m not actual sure I understand your question. If you want a delay to be triggered by 2 channels simply use mix->return and select a mix as an input. Just lift the faders on the selected mix master corresponding to the 2 channels you want to kick off the delay.
In my experience with my QU-24, A&H have pretty well thought of every possible scenario of FX Setup, so I would be very surprised if need to go off-board for a FX.