You don’t need to take the same number of levels in both classes, it’s common to take a single level from another class to get some proficiencies or class features.

The biggest negative is that you postpone cool class abilities, and you cannot get the best ones from higher levels. This also means that you could mess up the character if you don’t plan the build before starting, but this isn’t a big problem in this game as you can change everything later.

In the interview they mention a specific case of taking two spellcasting classes, that’s what I was wondering about.