Have you tried using just the one pedal? What happened?

Yes, midi thru could be relevant. Connect the pedal to midi in on one keyboard, then use a midi lead to connect thru on that device to midi in on the other.

But note, the midi sustain commands will always endeavour to apply to BOTH keyboards, at the same time. If you're using both keyboards for similar things, then this may be OK. If you're using one only of the keyboards, then again prob OK. If you're using both keyboards, but for quite different things, then you may NOT want sustain to be applying to both devices? In which case you may need the separate pedals, even if it's 'annoying'?

Geoff

As Geoff said, it all depends on your application. I assume the sustain pedal is a regular one that connects to «damper» or «sustain» input on your keyboards.
There are a number of situations that may arise, but I think the following approach should cover you:
First connect both pedals as you usually do.
Second, think of a scenario where you want your sustain to affect ONLY the keyboard you are playing, but not the other. Let's call this keyboard #1.
Now, when you play the OTHER keyboard, both of them are going to sound at the same time, and the sustain pedal will affect both of them whenever yiu use it. Let's call this keyboard #2.
Third, connect a midi cable from its MIDI OUT terminal to MIDI IN terminal in keyboard #1.
You're done.

Ref message from Hugo,

If the pedals connect to something other than midi, i.e. specifically <sustain> or <damper>, then they may not be sending NORMAL midi data. I'd have to check manuals for both devices to confirm that, and I don't have such manuals, and I don't know specifically which devices OP is using.

Your message above seems to suggest attaching both pedals to the respective machines as normal. You say that at THAT point, playing keyboard 2 will also play keyboard 1. How is that?

Then you suggest connecting midi out of Keyb 2 to midi in of keyb 1. When you have done THAT, only THEN will playing keyb 2 also play keyb 1. Surely?

The THRU connector passes on what ever has come in via midi IN. The OUT connector passes on the midi information generated by that specific device. IF (and I'm not certain about this being so) the data generated by the pedals IS midi, then you would need to use THRU.

Geoff

Note to OP.

I've looked at some info regarding Yamaha pedals. Don't know if this applies to your specific device, you've not said what it is.

This suggests that the pedal connector may be a standard 1/4" jack, and that it is NOT midi data.

Evidently, there are ways to turn the data from the pedal into midi, but this involves some sort of adaptor (and maybe a computer) to receive the data from the pedal and re-assign that data to a specific midi controller, and then send to the keyboard.

Once such a re-assign has been done, then the midi signal will go to both keyboards if they're connected THRU ---> IN.

I don;t know if this is possible with your setup, as you've not given any details.

Geoff

The standard behavior of synths, specially when we talk about YAMAHA, is that whenever a sustain pedal, which has been connected to «damper» or «sustain» inputs in the back of the unit, is used, two things happen: first, the sound of the synth is affected by this pedal and second, a MIDI message, specifically a Control change #64 is sent through its midi OUT.
If a cable is connected from this OUT to the MIDI IN connector on another synth, say, the Nord, this.message will arrive along with any MIDI NOTE ON and OFF messages, making this.second keyboard sound along with the first, including the action of the sustain pedal.
This is standard behavior. Some keyboards allow the user to modify this behavior, should they want to do something different.
So, yes, all you have to do is to connect a midi cable from midi out of, say, the YAMAHA synth, to the midi in of the Nord, and you have what you are looking for, provided that both synths are assigned to the same midi channel.
If you don't do the inverse connection, ie from Nord's midi out to YAMAHA midi in, then the sustain pedal connected to the Nord won't affect the YAMAHA.
You can just have only one pedal and connect it to the YAMAHA, in this example, because it will work, whether you play in that synth or in the Nord, as the Controller message related to sustain will still be sent.
Regards

If you use a scheme of a sustain pedal operating via MIDI, remember that the control of sustain (CC#64) is sent via channelized messages, and so you have to setup both keyboards to receive on the same MIDI channel.