Yes, if the Sonos Fives are set up as a stereo pair, the single line in on one of them will play in stereo with one being the left speaker, the other being the right speaker.

The key is that both speakers need to be in the same “room” in the Sonos software, and set up as a stereo pair. Alternately, you could just “group” the two speakers and while they’d play in sync, one would not be left, and the other right. 

Note there is no Sonos 5. There are PLAY:5s and Sonos Fives, though. 

First, let’s clarify some terms:

Grouping - Ad hoc grouping of Sonos devices to play the same content in sync.

Pairing - Semi-permanent linking of two like models where each device plays the left or right channel of a stereo signal.  Also called “stereo pairing”.

Bonding -  Semi-permanent linking of a Sub and/or surrounds to a single, paired, or TV capable Sonos device.

So, what you are describing is:

Two Fives grouped together to play a turntable via the Line-In on one of the Fives.  In this scenario, both Fives play both stereo channels. 

*OR*

Two fives paired together to play a turntable via the Line-In on one of the Fives. In this scenario, the Fives will play as a stereo pair; the Five designated as Left will play the left channel, and the one designated the Right will play the right channel.