If you are within range of the Wi-Fi network of your Sonos ecosystem, you can play Bluetooth audio from your Roam to any other grouped Sonos speaker on your Wi-Fi network including a second Roam.

I better option for an outdoor party would be the Sonos Move. A Roam is just too small to fill an outdoor area. It is more of a portable personal speaker.

Whilst at home, connected to WiFi, switch just one of the Roams to Bluetooth and play some audio to it from an App on the connected mobile device.

Goto the Sonos App System/Rooms tab (middle icon on main bottom bar) select the little icon on the Roam Room in the list …and goto the grouping page and group the Bluetooth playing speaker with the ‘other’ Roam. 

The Bluetooth audio should then play to both Roams.

If you have an android phone, you should be able to send audio via bluetooth to 2 separate speakers/headphones.  In this case, that would be two Roams. The Roams are not stereo paired, but from my personal testing, sound pretty good and cover a rather large space.

If you’re using Apple, perhaps something like this would help.   If you’re in WiFi range, the other suggestions are the way to go.

Ah! I figured out the Bluetooth pairing thanks to the comments here pointing me toward the right direction. So thanks for that!

Turns out I needed to activate a media setting in my Samsung Android phone to allow dual bluetooth pairing. This setting was hidden in an obscure location (bad UX, Samsung!) and I never would have figured it out without this video:

Yea, I thought that was rather weird too. I got lucky and stumbled across the dual pairing feature.