I don't have the Gemini - but I do have a "MODX" and an RD-2000. This would really apply to any MIDI-connected devices that support sustain MIDI messages.

Connect MIDI cable between MODX's MIDI OUT (5-pin DIN) to RD-2000 MIDI IN.

For Gemini, I would connect a MIDI cable between the RD-2000's MIDI THRU port to the Gemini's MIDI IN port. I'm assuming this device has standard MIDI connections. If it doesn't, then there are different configurations which would be needed (either adding a hardware translation device or use a tablet/computer and different routing -- won't cover that).

Connect sustain pedal to MODX's sustain port.

I powered up the RD-2000 and left the setup to the default power-on. Not much setup has to be done here.

MODX was configured in multi-channel MIDI mode, I/O mode=MIDI (not USB). Used CFX+FM EP as the MODX preset.

When I strike MODX keys, I heard the RD-2000 responding with its piano and other sounds. For a quick verification, I turned off MODX's tone generator with Local Control=OFF so all I heard was the RD-2000.

When I held the sustain pedal (only connected to MODX), the RD-2000 would sustain.

Anything else connected to this MIDI bus in a serial manner (cascaded through THRU) will pick up the notes and sustain as well.

Jason - Thanks for your suggestions. I did a similar thing but not quite what you described above. Although I have occasional use for layering the RD-2000 sounds with the MODX7 and/or Gemini, I would always like to be able to sustain all keyboards / sound sources with a single sustain pedal. Also, when I am layering piano sounds with anything else, I would be playing the RD-2000 keyboard. Thus the control flow needs to start with the RD-2000, So what I did was go from RD-2000 MIDI Out to MODX7 MIDI in. I also have the MODX7 MIDI Out cabled to the Gemini MIDI In. The single Sustain pedal is connected to the RD-2000. This successfully sustains the RD-2000 and the MODX7, along with playing the internal sounds of both (I have not figured-out how to get the sustain pedal messages sent to the MODX7 without the keyboard messages). With these connections, when I play the MODX7 keyboard, it also plays the Gemini via MIDI (this doubling is per my intent). Also the MODX7 dedicated sustain pedal sustains both the MODX7 and the Gemini (would rather not have to use a MODX7 dedicated sustain pedal). The RD-2000 sustain pedal sustains the RD-2000 and the MODX7 (as already stated); however, the RD-2000 sustain pedal does not sustain the Gemini.

If I switch the MODX7 MIDI Out port to operate as MIDI Thru, then the RD-2000 sustain pedal DOES sustain the Gemini (good) and the RD-2000 keyboard plays the Gemini (good sometimes); however the RD-2000 sustain pedal does not sustain the MODX7 (bad) nor does the RD-2000 keyboard play the MODX7 (bad sometimes).

I think maybe I need a MIDI merge box and/or a relay box that takes a sustain pedal input and runs several isolated sustain outputs, with both NO and NC switching. I have not been able to find this commercially, so I might have to build it. If anyone knows of such a relay box as an existing product, I would be happy to hear about it.

If you want RD-2000 to be the master, connect the RD-2000 MIDI OUT to MODX's MIDI IN. MODX's MIDI THRU to the Gemini.

I have not figured-out how to get the sustain pedal messages sent to the MODX7 without the keyboard messages

Having just sustain pass through while nothing else is not typical - I'll look to see if the RD-2000 has anything but I would think if you wanted this, it would require throwing more hardware at the setup.

The more typical solution would be to just get another sustain pedal and connect one pedal to your MODX and one to the RD-2000. It violates your goals - but this is how you would see the pros setup their rig typically.