2 HD Streams to ASIO Soundcard

Hi All.
So I've been determined to try and get some decent understanding of breakaway one functionality.
Don't get me wrong I'm enjoying the flexibility of the software.
My deal was trying to replicate the old Breakaway DJ, where it had 2 complete separate streams going out to  4 (2×2) channels, out to a hardware mixer and PA system.
(OTSAV guys may be interested!)
After a fair few late nights, and a few hot days, I've now achieved that goal.
System has now run a solid 8hrs non stop.

I've tweaked some more audio settings to get stable low jitter values on all channels.

Something I struggled to understand is the output assignments in both the ASIO and HD config screens.
Somehow this works with HD1 going to Ch 1/2 and HD2 going to Ch3/4, without me specifying it anywhere.

ASIO Setup

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