I was reminded of this when reading the title of Is the Doctor's Second Heart artificial? of a book I read as a child, somewhere in the late 1980s to the early 1990s in Ashland, Kentucky. It was a paperback, checked out from the library, that I think had a young boy on the cover, maybe with black hair, with lightning flashing in the background. It was of a pretty typical length for a paperback, somewhere between 100 and 200 pages.
The premise was that an alien boy is taken in by a family on Earth. I don't remember why he was on Earth or how he got there. I want to say he had dark hair with a white streak. His main peculiarity was that he had two hearts, something that I believe was verified in the book by them listening at his chest and hearing both beating. I think he had some ability to control electricity, part of what was depicted on the cover. Something happens which makes his heart stop (possibly tied to the aforementioned lightning) and they think he's dead, but then the second heart starts beating and he comes back to life).
Unfortunately, the only other thing I remember is a vague idea that the title was something like "The Boy with Two Hearts", but there's no such book on ISFDB, and it's not this one, which comes up when I Google search for that title.