No. One sub -- or Sub -- at a time. The system detects a third party sub connection. 

Why on earth would you want do this anyway? The crossover/phase requirements from the main speakers would inevitably be different, and two subs, especially from different manufacturers, are likely to fight. Phase cancellation could easily cause dropouts at various frequencies. 

Funny. You are sure?

I read this review of the new AMP which states that this is possible:

https://www.google.dk/amp/s/www.techhive.com/article/3403444/sonos-amp-review.amp.html

“If you have a large home theater, you can use both a Sonos Sub and a powered wired sub at the same time, and/or a second amp to power wired surrounds.“

I am moving to a new house we are building and the living room will be a rather large, open area. I am having in ceilings installed which will be plugged to two AMPs (fronts and rears). As I have a Sonos Sub I was planning on using that but my audio guy says it will be too small for the living room and that he ideally would recommend two mid-size subs for the area rather than one large. 
 

So the review made me think I might be able to find a 3rd party sub which could match my Sonos Sub. 

Point to Sonos documentation that says that and I’d be happy to retract. But two dissimilar subs, potentially with different crossover requirements, still sounds like an integration nightmare.

Is it possible to connect two identical 3rd party subs by splitting wires?

I don’t see why not. The crossover frequency would at least be the same for both.

With a large enough splitting scheme one could attach a zillion 3rd party subwoofers. Goodness of sound is a separate discussion.

Thank you for ridiculing me. Good stuff. Keep up the good spirit. Only people who know the right answer should be brave enough to ask a question. Will take note of this and get my answers straight before posting any questions again. 

It was not my intent to ridicule anyone. Introducing multiple subwoofers into a room must be done carefully for good results. Working out the details is a job for your audio guy.

A little late to the game but wanted to share my experience trying to do the same thing.  While possible to connect both a Sonus Sub and 3rd party sub to the same Sonos Amp, there are limitations and not worth it iMO.  The only way I found was to set the crossover for the Sonus Sub to 50Hz (lowest setting) and then connect my REL sub via high level input.  By doing so, you lose everything below 50Hz to the 3rd party sub.

I looked at doing this: 

In the end, I decided to buy another Amp and connect my REL sub via high-level then group the rooms when playing music and want fuller bass.  Not the cheapest solution, but it allowed me to easily test multiple locations for both subs to get the desired fullness. The room has high ceilings and is open on 2 sides.  Adding the REL sub made a huge difference.

Maybe someday Sonos will release a high/low device intended just for 3rd party subs (Similar to an Amp/Port that just does both high-level and low-level outbound signaling - no amp or inbound ports).  There’s a definite market for it, but probably not big enough for Sonos to consider. 

Would also be nice if Sonos allowed customers to connect more than one Sonos Sub to a “room”.  I think they would sell a ton more Subs but maybe they’re not interested in dealing with those that don’t know how to properly place and tune them.