This topic is not about Poly Aftertouch. This is about a simple 2 zones keyboard split :
Synth 1 : Aftertouch ON, left zone only. Synth 2 : Aftertouch OFF, right zone only.
Left zone keys trigger Synth 1 Aftertouch and don’t trigger right zone Aftertouch. Good.
Right zone keys don’t trigger Synth 2 Aftertouch. Good. But trigger left zone Aftertouch… Wrong
Therefore while I am playing both hands, my no Aftertouch right zone modulates left zone Aftertouch. 
Most keyboard beds are not able to notice where the aftertouch is applied to. So it can’t disable the aftertouch for a zone in order not to generate aftertouch received by the other zone. This is not a bug, but typical for most keyboards. Only a few polyphonic aftertouch keybeds do it right.
Why is this a problem? If you know the right zone has no AT assigned then why push into the keys to initiate AT? And if you’re getting inadvertent AT in the Left Zone while playing the keys in the right zone maybe you need to adjust the AT sensitivity.
Ok, thanks for the info. Because as velocity is set per synth (different velocities and no velocity apply in different split zones), I was under the impression that aftertouch, also set per synth, would react the same.
Because I have the most huge and heavy hands and as soon as I put those shovels on the keyboard, I’m already reaching aftertouch 
Jokes apart, my example was about differentiating 2 aftertouch zones.
Velocity has to be per key per definition. Even on non multitimbral keyboards, every key down will generate its own velocity value. While aftertouch is usually generated by one or only a few pressure sensors for the whole keybed.
I fully understand the situation, that’s why Moog One won’t get poly AT but will be able to receive midi poly AT in a further update.
For demonstration purpose, I found a (kind of) workaround for my example using Moog One as an expander only.
AT is enabled on both Synth 1 and 2, each Synth has its own midi channel and controlled from its own external keyboard :
Synth 1 controlled by keyboard A, Synth 2 controlled by keyboard B.
It works. Both Synths AT are now fully independent and not interacting with each other. Not using Moog One keyboard is a bit sad but as Synths midi channels can be saved per preset, it would be useful in a live setting.