Technically, if you just want it out as two mono’s the same then look at a Mult ( Multiple ) in the passive form Passive Multiple | Electronic things… and stuff
Would be better off as a buffered not passive mult though.
But why do you want twin mono? what’s your plan?
Yes, you can connect the output signal to both tip and ring to put it on both stereo channels. Per the schematic here STUFF - LOOK MUM NO COMPUTER the output’s just direct from an op amp — not even a 1k current limiting resistor, surprisingly; I’d advocate adding that on both the tip and ring to make sure shorting one doesn’t short the other or the op amp, not that it’s likely to be a disaster if it does. Anyway, splitting an op amp output to two destinations is not a problem. Buffering the two separately would be nicer but not really required. People do this all the time, sending one signal to two destinations:
I just want the sound to come from both speakers of my soundsystem. No need for it to be anything more than the same mono signal from both left and right speakers
Yes, that was my reasoning as well, doing it on the socket would save me from buying a mono-to-stereo cable wich in essence is the same.
Sorry, I misunderstood the question, because for me outputting the same mono signal twice was not “real” stereo, moreover like this you will not be able to separate by pan the different signals entering the module.
Only 2 identical mono signals are obtained.
but the question was rather on the stereo jack itself, and @analogoutput very well understood 
even a mono sound should come out of the 2 speakers, did I miss something ?
If you send a mono signal on only the tip it’ll appear on only one stereo channel in the sound system so will be heard on only one speaker, unless your sound system has some way to pan or convert stereo to mono.
Before the speakers logically there is an amp (or else the speakers are themselves amplified like studio monitoring), the signal synth cannot be entered directly into the amp, so it is necessary to an intermediate element such as a mixer, an audio PC interface … no ?
@Arie what is your sound system setup ?
Like everything about this build the soundsystem and especially the way to connect it is still somewhat up in the air. I have a system consisting of an amp, 2 speakers and a sub. Main input for the amp is 2 channel RCA. It could go via a PC, but at first I was thinking of using the mini mixer as final step to bring together different patches and the send it to the sound system, which is where I thought I needed to ‘clone’ the mono channel to be able to hear it over both speakers.
As soon as more gets built I guess I will just try stuff out. As of now I have only completed (but not tested or tuned) a single 1222 VCO.
As a later project you could consider an output module that takes two or more inputs, mixes and pans them into a stereo field, attenuates them to line level, and sends them to two mono output jacks (or a stereo one, but two mono is more versatile). Maybe also via a headphone amplifier to a headphone jack. There are a few such designs around — I built one, though the mechanical design was so stupid I haven’t published it.
