You cannot add speakers as the front left and right with the Beam as the center.  The Beam, like all soundbars, already has the front left/center/right speakers.  You can position the Play:1’s as surrounds (behind and to the left/right of the main listening position) and add them that way.  See this link:

https://support.sonos.com/en-us/article/add-surround-speakers-to-a-sonos-home-theater

The Beam already has right, center and left speakers inside it, there is no way in the Sonos system to add additional right and left speakers to it, only surround speakers. You would need to move those PLAY:1s to behind you, and go into the Sonos Beam’s room, and add them as surrounds.

Sonos does not make a ‘Center’ speaker. All of their sound bar speakers contain left, center, and right in the single enclosure. 
 

edit…read the post previous to mine ;)

Thank you both for correcting my misunderstanding.

Followed the link, after carrying out a factory reset and reconnection of both speakers.

I now have them as surround speakers.

Merry Christmas

Don