Add a second Era and then stereo-pair them. The Eras will then “share” the data playing left stereo channel to left speaker and right to right. 
They will need a line-level signal from a pre-amp, and the Sonos adaptor to input through the usb-c port. From your question, your tt may have that output, and you may already have the adaptor. 

Hi Nika

thanks for your feedback…but I dont understand yet !

The two era 300 already work in stereo if I play music from music streaming (spotify etc).

When I use the turntable instead, the music is only heard from the speaker connected in line in.
 

What should I do to hear the music on both speakers in stereo format?
 

My turntable has the outputs as shown in the photo I attach. The model is project t1 and I don't think it is pre-amplified.

Thanks a lot for your support.

Ahh, you’re now asking different questions and telling us a bit more - like you have got them set up as a stereo pair. But just to be sure: are they both set in one Sonos room as a stereo pair, or two rooms which you’ve grouped? 
 

The Line Out you’re (correctly) using is the  amplified output; the Phono connections are non-amplified. 
 

If you’re only getting sound on one Era, make sure the cables are connected correctly. Pull them out and plug them in again. If still no joy plug the red connector into the right socket of line out, and the white into the left. If sound now comes out of only your other Era, there’s a problem with (a) the turntable, (b) the stylus connections, (c) the cable, or (d) the Sonos adaptor. 

Hi Nika

both sonos era 300 set in one sonos room as stereo pair.

to be very precise, both speakers are connected to the same wifi line but only one speaker is connected line in with sonos adapter to the turntable. Is is right or both have to be connected to the turntable line in ? If yes, how can I connect both line in if the tt have only 1 line out with 2 channel ?

thanks

You only need the one adaptor and it can be connected to either Era; the Sonos devices take care of sending left signal to left speaker and right signal to the right. 
 

It would therefore seem your Sonos kit is set up correctly (unless the adaptor is faulty). 
 

What happens when you try the things I already suggested?

Hi Nika, 

you re the best !!!!!

The cables were not connected rightly!

now everything is ok and the sound is brilliant !

thanks you very much !!!