How MSX pseudo-stereo works?

The music of Unknown Reality is made in Moonblaster 1.4. That goes for the FM music, the samples are done in Sampbox.
Moonblaster supports MSX-Audio and MSX-Music at the same time. The developer expects that the musician and the listener has an output on each of the cartridges and each connected to a different speaker (EG, Audio left and Music right).
MSX-Music can play 6 FM plus 5 drum channels or 9 FM channels. Moonblaster can switch between both modes. To reach out to the musician, Moonsoft has added PSG drums to fill up the gap.
This PSG drum hasn't been used that much as they sound like dry farts, though Compjoetania did use them for Unknown Reality. My guess is to have some drum in the least favorite setup possible: MSX-Music only and monitor only. That way both chips are combined and there's some kind of rhythm.
If the desired setup is available, the ADPCM drums will play on the MSX-Audio speaker and the MSX-Music speaker will have no rhythm (unless you own an FM-Stereo PAK which has a line in to join the PSG with its output).
This is mostly intended.
As you develop hardware, and cloned PSG is already available in MFRSCC+SD (dunno about Gr8net) eventually PSG can be mixed to MSX-Music.
It might be nice to have this PSG/MSX-Music mix coming out of a cartridge's audio output because there are quite some Japanese games which support both at the same time.

I think it is not about making a PSG but gr8tnet gets an OPL.
I think the question is, there are two sound cartridges, what are their sound connectors.
I hope the OPL interrupts work same in gr8net. For programs for off the shelf hardware. I need IM2. I got 257 bytes table for noisy databuses.

https://www.msx.org/wiki/MoonSound
"The MoonSound comes with a cable (mini jack to 2 × cinch; stereo) to connect it to an amplifier"
a stereo mic out connector?

https://www.msx.org/wiki/Panasoft_FM-PAC#Specifications
"Mono sound output"

maybe the PSG slot pin is not used?

As you know current version of GR8NET is mono and both OPLL and OPL are talking onto the same channel, going to MSX output.

I am thinking to make stereo out, but can not decide how should I wire things so that existing software work properly as intended. I want to make it correctly from the very start, and that's why I ask you. I want to avoid resoldering in the field (e.g. rewiring channels), and avoid using any switching equipment (either switches or analog multiplexers).

Imagine the following setup:

• DAC has two audio outputs, "left" and "right";
• Each output is being separated into two streams -
  
  [1] into 2.5 mm phone/line out jack, and
  [2] into MSX slot pin 49 (going to MSX internal mixer), where are being mixed with machine's PSG

First issue, should I have both channels to go to pin 49? Or only one must be mixed with machine's internal PSG. According to @Meits and @NYYRIKKI I should put OPLL through, let's say, "left" channel to be fed into machine, while "right" channel is fully dedicated for the jack output only. Thus only one channel will be routed to the MSX slot pin 49.

However there're other setups possible:

• I can put PSG into the GR8NET (if space permits) to mix with the channel of your software choice, do not output any of GR8NET audio into the MSX slot connector; But it is a big question if it is worth in terms of software support and quality of sound; PSG will still sound like "dry farts", and audio generated will not be heard through MSX speaker;
• route both channels into MSX slot, thus it will be mess through MSX speaker, but with PSG, however OPL and OPLL will be clean through the jack's dedicated channel.

There were three variations of MSX-Music produced in the Netherlands:

1. Mono (most common), often no cartridge jack
2. Pseudo-stereo with a small delay on the other channel of the cartridge jack to get a "stereo chorus" effect
3. Stereo (FM-Stereo-Pak) which outputs the FM channels on the left and the FM-drums on the right of the cartridge jack output (the YM2413 has separate outputs for them)

I’ve never heard the 2nd variation, but I have the 3rd and I’m not a big fan of that effect . However for mixing / post-processing the music it might be nice. Btw, I’m not sure any of them outputs PSG on the cartridge jack?

The MoonBlaster 1.4 stereo is as Meits describes it, but I’m not sure if the MSX-Audio or the MSX-Music is supposed to go on the left channel. Both output in mono to the MSX, of course.

And MB 1.4 PSG drums support, I didn’t even know about that! Kinda neat though, too bad it wasn’t used so much and didn’t include PSG tone channel support, because I love love Micro Cabin-style music . Trilo Tracker FM also supports this OPLL+PSG combination. But I don’t know if it was ever seriously used in a stereo configuration.

What sounds like maybe a good setup is, output both FM chips to the MSX in mono, output them to the jack in stereo, and output a GR8NET PSG to the jack on both channels. Make the FM chips mono/stereo a settting, maybe, so they can also both output to the jack in mono (for non-stereo applications).

Mb 1.4 its psg support was really bad.
@grauw: that fm'stereo' pack was a really bad idea.