I am just about done writing the first book in a series. I have an idea for the next book in that series (let's call it "book a"), but I also have another idea brewing in my mind which takes place in another universe (let's call it "book b"). I want to write "book b" badly, but I feel I should get my first series over and done with. I feel like starting another before then might confuse me. To be clear, I do enjoy writing about the universe of "book a" a lot, so I'm not quite sure what to do right now.
Is it advisable to begin writing a book in an unrelated series before finishing my first series?
If my wording was too confusing, let me know and I'll try to help. :)