In a nutshell if I select 2 subwoofers in setup - it takes me to SW Level Match (where one sub is adjustable for ~75db, but the other does nothing if adjusting gain/volume. Switched cables to preouts did the same on other sub.
So basically the only way so far is hooking 2nd sub up as slave (mimics) or using Y-splitter which is what I think internally the Denon pre-outs are doing.
The examples above have been tried using 2 subs in setup, I have to skip SW Level Adjust to finish calibrataion in Audyssey XT32. Does same thing in MultEQ iOS app.
Have 2 new sub cables arriving tommorrow.