Did as instructed. Same error message
The problem only arises if the subs are not equidistant from the MLP. If they are equidistant, then the way ARC handles them is fine - it will just see them as a single sub and EQ "it" accordingly. If the subs are at significantly different distances from the MLP, things are trickier - Jerry is used to Audyssey XT32+SUbEQ HT which allows for the setting of the delays (distances) and levels of dual subs separately, then going on to EQ them as one sub.
The problem with the suggestion in the manual is that not all subs have continuously adjustable phase controls (my Submersives for example) and have a simple phase reversal switch. For those subs, tuning them for phase to optimise the splice by using the onboard sub controls is not possible and the only way to adjust them is to change the delays (distances) in the AVR. This is only possible in AVRs which allow for dual subs with individual settings. Because Anthem doesn't allow for this, it is a limitation.
It is possible to work around this limitation by introducing a miniDSP between the processor and amplifiers (this assumes external amplifiers) and using that to set levels and delays for each sub individually, but this is a fairly advanced procedure and not everyone has the knowledge, or the inclination, to attempt it. It is also at odds with the 'automated' nature of room EQ systems found in modern AVRs.
While I see it as a limitation for some dual sub users, hopefully, the other benefits of ARC and its flexibility for user-adjustments, will compensate.
It would be great if you could find that post. I am sure Jerry will find it useful (congrats BTW Jerry on your purchase) and so would I, even though my subs are, to all intents and purposes, equidistant from the MLP.
The dealbreaker for me, unfortunately, is that I don't want to lose my Height channels and so the x10 series is no good for me. That decision to lose Heights in favour of useless passive biamping really niggles me.
I did consider buying the x00 series but I am reluctant to take a step backwards with ARC as that would be my primary motivation in going the Anthem route anyway. Jerry and I are old buddies, so I am very much looking forward to his findings wrt to ARC vs XT32 and I know he will back up anything he says with some nice, objective REW graphs too. Should be a very interesting few weeks.