If a Roam is both connected to BT and the local WiFi and it is playing BT audio, it can be grouped with any/all Sonos rooms/devices in its Household using the Sonos App or SVC …and all devices will play that same BT audio in sync. That’s upto 32 speakers in total (including the Roam). 

Not on Wi-Fi, only on Bluetooth. I’d like to have two Roam speakers set up as a room pair streaming from Bluetooth on my boat. I currently use Two Wonderbooms by Ultimate Ears and the pair together and play in stereo. I’d like to do the same with two Roam speakers. It’s Time to replace the Wonderbooms. 

That’s right I’m talking about playing the audio using Bluetooth. I use it when camping and do the grouping over a MiFi hotspot, but all the Bluetooth music is sent from my iOS Apps (Apple Music/Deezer/YouTube etc.) which are playing over the phones Bluetooth connection. I understand that upto 32 speakers can be used in the group,  but I have only ever personally played the BT music to two Roams and a Move at the same time when away from Home. It works fine.
 

I also sometimes use a Sonru TX Bluetooth hub (rechargeable) and that allows mobile playback to two Roams too in perfect sync. The other method though over MiFi does allow stereo pairing, so I tend to use that mostly.