When a man is cut into two symmetrical halves completed with mechanical prosthesis, the two sides fight each other to become a full person again.

This sounds really interesting, he fights himself! I like it 🙂

To make it more interesting, I would make one of the halfes the main character and the other one more of a antagonist. So maybe something like:
“When a man is cut into two symmetrical halves completed with mechanical prosthesis and one of the halves becomes rabid, the other half must fight him and make them into one person agian.”

Or make the person(s) who cut the man in two the antagonist and the halves must work together? to become one person again. That would be more of a “get to know my own good and bad sides” – movie, where the main character has some revelation of? that he maybe is quite a mean person and changes this.

Personally, I like the first one better, where he must fight against himself 🙂

Hope this helped!