In my case, there are two mono sockets. So if you want it to be stereo, you would have to route the signal from the synth to both inputs (line and XLR). That would work.

Hello Tacheles. I had to return the audio interface because it was making a lot of humming noises. I don't know whether I received a defective device or whether it is not working properly in conjunction with the M2 CPU on my Apple laptop.

I use it with my Moog Sub37, but it only runs mono. A simple setup would look like this: USB to the synth (to send/receive MIDI directly from the DAW/garage tape), Synth-Output-L (is the mono channel) then into the Scarlett, and then Scarlett-USB into the computer to record the audio channel.
For stereo there would be a workaround via line + XLR (as already commented here), but it only has disadvantages due to different impedances and because of the balanced input and you would probably need a DI box. I would then go for the 2i2.

I think with an RCF cable plugged into the rear port you can connect it, all the way I'm not sure if the sound will be in stereo, on the front it has 2 knobs (1-2) to adjust the volume, I use it mainly through production software I'm not sure