ARC Ultra and two sub mini’s

Sonos software. 

Exactly. That is why I’m asking for a “technical” limitation that precludes it. 

There is no scientific evidence that one sub is better than two and there are viable reasons there are multiple subs in dedicated Home Theater setups and movie theaters. 

As I suggested, the Sonos software precludes this. I understand your argument, but Sonos makes their own decisions. You could equally question many other choices they’ve made, but never explained ‘why’, we in this community can only guess. 

And further, it may be (and at least I suspect) not a technical limitation, but a marketing decision. 

A BULLS*** imposed limitation. 

Moderator Note: Modified in accordance with the Community Code of Conduct.

There are development and support costs associated with the feature. I can imagine this feature was omitted in order to keep costs lower — allowing a lower selling price.

Nonsense. If it works with Sub 3’s & 4’s absolutely no reason it would not work with Sub mini’s. There is nothing needed but an LFE signal sent by ARC Ultra to the subs at which point subs internal electronics convert signal to audio in proper phase and frequencies. 

Who cares what the reason is?  You can’t do it, and all the rationalization (or arguing with your fellow users here) isn’t going to change that.