I am thinking about twin star system with 2 stars (similar in mass)
What would be a shape of planet orbit in these system? Is it possible existing 8-shaped orbit? Is it possible existing of planet in the centre of gravity in this system?

No, these configurations are not stable.
If a planet would be part of the system, it most probably would orbit around one of the stars.
However, it's widely accepted that dual star systems have no planets.

I agree with the master expert. The fields of gravity would be unstable thus making a path of orbit impossible. BUT on the other hand if the stars gravity pull was equal this might create a equal pull to both stars and if a "planet" got stuck in this it could remain their.