This has been requested previously and there are a few threads on the same topic where the Sonos Staff have responded and forwarded the request onto the Sonos Engineers… here’s one example

At the moment there isn’t anything to say this will happen and Sonos very rarely comment on their development roadmap. It’s a case of waiting to see if this may happen, but it is a feature request going back some years.

A little waste, not unlocking this potential. I assume its an easy option to add in the S2.

S2 has the same restrictions. The only ‘room’ that can contain two Subs remains a home theater room, one of the Subs must be a gen 3. There is no way in S2 to have two subs connected to two Fives in a ‘music’ room. 

Maybe the dual Subs will be introduced with the Era-500’s, if those speakers are in the pipeline and will be replacing the Fives, as some have been speculating.

We shall just have to wait and see what happens - we can only guess at these things.