No, you cannot (with the Sonos app - it may be possible with a third-party app). 

Maybe swap the mini for a full sized Sub?

I'm very happy with my Era 300 Pair and an older full sized Sub.

Thanks a lot for the reply but what is the point of using a third party app, it is Sonos hardware, it works with the Sonos app.

I have a Gen 3 sub in the bedroom, maybe I should swap the subs with the Sonos sub mini in the living room?

Try it and let your ears decide which is better. 

The question was asked before, and I only answered “no”; then someone said a third party app would allow it, which I didn’t know at that time. So, for accuracy I felt my answer this time was appropriate. 

Bonding two Subs is only possible in Home Theatre configurations.

Sonosequencr can create a dual sub setup with two Sub Minis, but again that only relates to a home theatre configuration.

Do you know the name of this app?

Thank you so much dude, you're wonderful.

There are several things third party apps enable that the Sonos app doesn't. The risk is that at some point Sonos will make changes that no longer allow the unsupported things.

How much risk is hard to say, it could be very low or some marketing guy in a $1000 suit could get unhappy and they could be gone tomorrow.

I'd say use them if they allow you to use what you now have, and deal with any issues if/when they happen. But I'd hesitate to buy new stuff that depended on them being available. 

Do you have a list of third-party applications by any chance that allow you to manage more settings or configurations than the official application, other than sonosequencr please?

 For fun, I added my Sonos Sub Gen 3 to my Era 300 X2 + Sonos Sub Mini setup, I haven't tried it but the setup works anyway and it's not in home theater.

That’s news to me. I’ve never heard of anyone adding ‘dual subs’ to a stereo pair.

I just use the Sonos app now, haven't kept track of the other stuff but for Apple or Android a simple search for Sonos should find anything there.