Buy more speakers? 
 

or, more seriously, I think it’s only the Sub gen3 that allows multiple subs to a soundbar. Sub mini is limited to one-per-master-speaker. 

I do not think you can ad two Sub Mini’s to one “room” in Sonos, as you can with non-mini Subs (as long as at least one of them is a Gen 3).

I thought they had that option, since they got the option to reverse phase. So i’m stuck with an useless subwoofer

It’s not useless, just surplus to your current needs. Or, return it. 

I’ll try, but don’t know if they will accept since i opened the box

Hi @nofearc 

Welcome to the Sonos Community!

If you bought direct from www.sonos.com, we don’t mind if you opened the box - how else could you try it? I see you are in the EU - that means you have a 100 day no-questions-asked money-back-guarantee period (again, if you buy direct). You can start the process yourself at https:/www.sonos.com/mbg.

Stores may have a different refund policy.

I hope this helps.