Thank you Fetcher, I realize that my electronics skills really suck, I've tried with a diode that I've taken from a broken voltmeter (1N4007, that should handle up to 100V IIRC), and now it works
I'm still not sure whether I will use it or not in real world, at this moment I've tried it just with the wheel spinning on air and two independent throttles, one for each controller. The greatest inconvenience I've found so far with this set-up is that the regen braking only takes place when I operate on the regen throttle immediately just after having released the main throttle, otherwise the regen controller voltage quickly drops below LVC and then it's not possible to regen any more until I accelerate again. Is this an expected behaviour?
This issue could be partially resolved by using a commutator as a brake switch, connected to both controllers at the same time but with opposite configuration, so that when the main controller is braked the regen one isn't, and the other way round. But there's still the problem that in real practise, if for some reason you release the brake you will not be able to regen again until having throttled (unless your speed is enough to provide a voltage higher than the LVC).
Aside from this, there's also the issue about how to limit the regen current, the simplest solution would be to attach a potentiometer in place of the regen throttle and give it a fixed setting. The coolest one would be to attach the same throttle in parallel to both controllers (with the avoiding both to operate at the same time), and introduce a limit for the regen controller, I can think two way of doing that:
a) Try to make a pull-down circuit that limits the maximum throttle signal that the regen controller will receive. (I would need help with this one)
b) To shave the shunt of the regen controller until the maximum current is acceptable.
Thanks again for your appreciations.