What do you mean? I was quite sure people that work on anime can work for multiple studios at the same overlapping point in time. Additionally, weve already seen that someone doing a shonen jump manga for Shueisha (Narita) was still penning stories for ASCII Media Works (DRRR SH!), so Im confused by your "you cant go emplyoing other employees workers." line. So unless ASCII Media Works of Kadowaka is now owned by Shueisha, your comment is a bit confusing and nonfactual.
Most Animators are freelancer and not fixed employers.
About manga authors, normally they have a exclusive contract to the company, but can be made extraordinary agreements where one person can work for various companies. It can be that he is a freelancer and only the respective works are exclusive of the companies, or can be that he still works for one company and the other companies have to pay royalties to the effective one.
Even this extraordinary contracts normally only work for the same field and different works, so in this case mangaka and not Hunter x Hunter.
We have cases of Toriyama that isn't an employer of Shueisha anymore(I believe), but Dragon Ball is still a Shueisha and Toei Franchise.
Takehiko Inoue that is doing Real for Shueisha and Vagabond for Kodansha, though I don't really know what is situation with both publishers. I would assume that right now he is a freelancer author.(Given that he started doing Vagabond 2 years after Slam Dunk, and for what Bakuman said you're fired after 1 or 2 years without having a new serialized work in Shonen Jump).
Kajiwara Ikki, creator of Ashita no Joe, that was doing at the same time works for Kodansha and other rival publishers, though he had to use a pseudonym to achieve that(so outside of the law XD).
Note that all the cases of working for different companies at the same time also means doing different works, not the same. So unless Shueisha fires Togashi or Togashi leaves Shueisha, and Togashi is able to get full rights of Hunter x Hunter, him getting employed to Madhouse(or any other anime company) with Hunter x Hunter will not happen.
Do you really believe that Shueisha that left Togashi take as many hiatus as he wants, never ending the series nor changing from Magazines will allow Hunter x Hunter to slip through their fingers?