No, you cannot Add a Sub Mini to a room that already has a Sub. 

Oh ! Fine then i guess i will add a second sub gen3

Thank you for you quick answer

Have a nice evening !

I just purchased a sub mini but was unaware it would not pair with the sub. For the price point I was greatly disappointed in this. I have returned the sub mini but will not purchase the standard sub which is several hundred dollars more. I currently have 2 play 5's 1 arc, 1 playbar, and several play 3's. I love the quality but Sonos continues to limit what you can do with their speakers and where you can place them.  

Quality of the products are top tier, but I’m often frustrated with the limits of configurations. Such as, I have an Arc, G3 Sub, & two Ones in my home theater. Sound is incredible, and Sonos gives the option to add a second sub….. buuuuut it can’t be a Sub mini? What?! Why? Additionally, I’d like to add two more Ones, on the sides for a 7:1 set up, maybe even put to Fives in the back and move the Ones to the side….. buuuuut can’t do that either.

Sonos is also about ease of use. More (wireless) speakers means more complexity and more things that can go wrong. Adding 7.1 would ad cost (processing power for Amp and soundbars that act as coördinator for surround) for a probably small percentage of users. Creating the possibility for the use of two different (in size and therefore in sound) would if possible cater to an even smaller audience (and result in less sales of a more expensive Sub).