@domo Re: being a member of both, I'm actually not sure! If there would be a problem with it, I would expect it to be on the side of the 'formal' union though, as they tend to operate within specific legal frameworks, so probably best to check with FNV.
As for tech unions, the closest I know of would be https://techwerkers.nl/en/, but I think they are probably a bit less radical than eg. Horeca United is. Haven't run across any overtly radical ones yet 😕