uh…*the correct Y-cable*… they don’t *have* to be mono

Totally agree this could be an EH thing, but till then.

If the control voltage is on the tip why on earth do you think you can’t split it with a mono Y-cable?
Basically that’s simple enough with the 8 step. Just split the output.
For an active expression pedal…the same.

Now if you want to use a passive exp ped then it’s a different game.
Sockets to take exp peds, like those on EH and Moog Gear, provide 5V on the ring.
So you’d want to get that 5V to the exp ped so it can be divided by the pot.
You need at least one stereo jack on your Y-cable, and to not wire anything to short out.
Depends on the exact way each of your pedals works, but a straightforward Y-cable using all stereo jacks and cable might be a place to start. Given that the stomp boxes are both 5V on ring then there’s no need for this “channel isolation” that you speak of. Ring carries 5V, tip carries your CV, shield is ground.

You can try using one stompbox to provide the 5V, but whether it’s capable of driving the whole setup will be an experiment, and you’ll have to solder up a custom Y-Cable with correct connections.

The piece of kit you can buy is the Moog CP-251.
That will allow you to use a single exp pedal to control 3 or 4 pedals.
With inversion should you require it, although you’ll have to deal with the fact it’s based on a +5 to -5V standard which means messing with DC offset form it’s mixer.
..and some possibility for different ranges.

If you can find/afford a Moog mp-201 then that has/had 4 channels of very configurable voltage control, and a footpedal.

Brass9

Trumpet
Mute Trumpet
Sax
Tower of power ( 4 part harmony tpt-sax-tpt=sax from low to high)
Oboe
Clarinet
Flute
French Horn
Ensemble( Bassoon to start with, but as you turn up a control it first brings in flute to top notes, then fills in any mid harmony with fr horn clarinet oboe)…or simpler, but you get the idea.