Question: I'm currently working on a book with two protagonists and switching the perspective between them. They have quite different plotlines, but actually they're the same person. English is not my first language, and in my language a subject always has a gender-neutral word. But I'm afraid my readers will feel cheated by the revelation/twist. 

How to avoid this and make it believable?

Answer: 
**Do you mean a Jekyll/Hyde plot?** Such a plot twist needs some amount of foreshadowing, so that the savvy reader might suspect, while the less savvy reader would have a moment of "Aha! now it all makes sense!"

Such foreshadowing can come in the form of information that Jekyll receives and Hyde responds to, Hyde doing things that would be in one way or another beneficial to Jekyll but Jekyll would never do, Jekyll avoiding conversations about Hyde, etc. (All of this works regardless of whether they're a good/evil duo like Dr. Jekyll & Mr. Hyde, or any other distinction between them.)

Another important point would be that they can't be at two places at once. So Jekyll might be unable to make an appointment, for example, with no good explanation, the real reason being that he's busy as Hyde.
One way to do this would be the way the movie "Fight Club" did it.

In that movie the main character is a passive person who meets a very active person. This person convinces him to set up a fight club where people come to voluntarily fight each other. Things get out of control, and little clues start building up and the MC realizes almost too late that he is both characters.

Another way to do this is to have the MC act out two different lives or parts of his/her life. For example, the character could be a young woman in love in one plot line, and the leader of a revolution in another.

I think one key to doing this without turning off the reader is to leave little hints, especially if the character isn't doing this on purpose or by insanity. The character may just be very focused and not normally think of the other part of their life.

For example, the young woman in love is reading the paper and is happy that the revolution is winning instead of thinking of them as terrorists. Perhaps when reading she knows just a bit more than she should know from the article, or contradicts a TV report to her family. Perhaps the leader of the revolution is wearing a piece of jewelry that was given to the young woman by her husband.

Another key is to have the plot lines eventually converge. Perhaps the antagonist realizes who the leader of the revolution is and starts a move to arrest everybody in the young lady's house, or to bomb it.