Question: If you have write one series like where people fight monsters and demons and then you write another series where not the same type of people but similar ones fight like vampire or werewolves only and they take place in the same world and you're thinking about doing a crossover of the 2 series for like another series where the main characters of each would have to come together for some reason. Is that a good idea? How often does something like work? I know with the Percy Jackson series and the Kane chronicles people wanted a crossover of the 2 and the author is working on that but the books are shorter than normal books.
If you haven't written either series yet, then I wouldn't worry about it. Your focus should be first on writing and selling one book. If you're lucky enough to do that, it will take another stroke of luck (as well as talent) to have the chance to write a series.
Years from now, if you are an established writer with two popular series under your belt, a crossover might be an easy way to boost sales of your backlist. The hope will be that fans of one series will read the crossover and then buy the books in the second series. Ditto for fans of the second series. Those who bought a trilogy plus the crossover will then go on to buy another trilogy for a total of seven.
Nonetheless, I appreciate that you are trying to think ahead, and there's nothing wrong with that (unless you get so involved with planning series that you don't get the first book written).
Incidentally, the crossover technique is one that helped make Marvel comics very big in the 1970s. There would be not only crossover series but also story lines that crossed over several series so that readers who wanted to read the complete stories would have to buy issues of series they never bought before. (Rather like how understanding the TV series Agents of Shield
requires seeing the Avengers movies, which in turn gives you reason to see the movies of each individual hero.)
But I digress.