Found a solution myself (for a Sonos Play1:, Sonos Play3: and Sonos Play5: Stereo Pair):
If you have created a Sonos Stereo Pair (with the Sonos SW) play any of your tracks to see if it works.
Let the song play and switch over to JRiver. Identify the Sonos Box playing your song and rename it "..... Main".
Now, if you stream your music only to that speaker the other will follow automatically.
Do not link the players. It will not work.
Hope it works for you too.
Regards, Peter