When an author envisions a book as part of a series it’s natural to finish the first book and dive right into writing the second book, or the sequel. Let me explain why I don’t think this is an effective business plan.
No matter how you look at it, a sequel is dependent on the first book. It’s why you call it a sequel or why you refer to it as the second book in the series. By writing the sequel before you’ve sold the first book you’ve potentially limited yourself from selling two books, especially if rejections on the first book are based on the idea or the hook and not the writing. A second book with the same hook isn’t likely to sell any better.
My suggestion is instead of focusing on the series you’ve planned, create another series. Should you sell that first book, which I certainly hope you do, it will be easy to circle back and continue writing the series. If you don’t sell it, or it takes a super long time, you now have something fresh and new to start submitting instead of struggling with who to submit the sequel to after they’ve already rejected the first book.