Finding success by writing in multiple genres is a matter of commitment. Whether that's commitment to maintaining different personas for each pseudonym you use for each different genre, or commitment to cross-promoting and making sure your audience is aware of and willing to try your work in other arenas all published under the same name, it's a lot harder to keep multiple plates spinning instead of just one... but it can definitely be done if you concentrate and commit to keeping them all going at once.

The degree of difficulty increases the farther apart the genres are. If you write fantasy and YA science fiction, that's no so much of a stretch that you'll be shedding "crossover" readers in nearly as great numbers as if you write, say, YA science fiction and Regency romance. If your different genres include some of the same elements, it will be considerably smoother sailing than if you're trying to write in two genres that are so disparate you only have a handful of readers willing to explore both types of stories. J.K. Rowling went from children's fantasy to adult crime novels and decided to use a pseudonym to avoid confusion. Brandon Sanderson, on the other hand, uses his name name for all his work even though Steelheart is in the YA section and The Way of Kings is in the regular sci-fi/fantasy section.

There's a difference between tonal genres and content genres, and the latter is much harder to crossover. Content genres are those where the genre conventions are related to setting, story elements, or character archetypes: westerns, fantasy, science fiction, war, mystery, etc. These are genres whose readers come to expect to find certain familiar tropes in the books they read. Tonal genres, on the other hand, are ones where those specific tropes and conventions don't matter as much as the atmosphere or emotions the story creates: horror/thriller, romance, etc. It's easier to crossover and write multiple genres if you stay under the general umbrella of horror, for example, than it is to write in multiple genres where each one has a set of character, story, and setting conventions that don't overlap. Stephen King has made a remarkable career for himself being labeled as a horror writer, even though - depending on the story - his work contains could be classified as fantasy, science fiction, drama, paranomal, etc. And yet they reside in the same section of the bookstore because the overall tone and identity of his work is horror.