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.
Well, perhaps focus on different sides of the character's life. If one plot revolves around family/friends/a love interest and the other plot revolves around a mystery, let the different plots bring forth different sides of your main character. 

One strategy could also be to let other characters call your main character different nicknames depending on which plot is evolving - One person can have one nickname in one group and another name in another. 

Futhermore, if your main character is placed in different scenarios in the different plots (which we could assume they are), they would also mentally think differently about their experiences and the feelings connected to them.