You would connect a stereo cable into one PLAY:5, and assuming the PLAY:5s are set up as a stereo pair, and not just grouped together, you would get a stereo signal. Right out of one, left out of the other. Essentially, you want RCA stereo to 3.5mm stereo cable, along the lines of this. 

Thanks, Bruce! They are set up that way. Sounds like I shouldn’t have a problem. Thanks for your response - really appreciate it.

I bought the Pro-Ject T1 Phono SB Turntable and I have two Play:5’s.  I’m worried I don’t have enough connections in the back of my Pro-Ject T1 Phono SB turntable to connect two Play 5’s.  Am I worried for no reason or do I need to simply buy a special cable?

You only need to connect your turntable to one Play:5. Read more here: https://support.sonos.com/s/article/3548?language=en_US

Great.  So I only need to plug the turntable into ONE Play:5...and the music willl play out of BOTH Play:5’s in stereo - because they’re both connected to WiFi?  Am I understanding that correctly?

Yes, you are correct. Be sure to set up the two Play:5s as a stereo pair if you haven’t already done so.

Thank you!