I've been hearing a lot of newbies on various sites talking about their manuscripts being way too long, so they then split those behemoths into two or more books.
Uh, well, the main issue with this is, if your first book doesn't sell well, why would a publisher roll out the next one? Would you really like your readers to be stuck with an ending that's no more satisfying than cotton candy?
And also, when readers begin your book, you've invited them into a world that poses questions in their mind. You are obligated to answer them. You can leave threads to be picked up in another book, sure, but you must wrap up the story you're telling in some tidy, satisfactory way. If you're planning two or more books during your writing process, the whole structure of each novel will be different than if you split one and magically make two. Each book needs to have its own arc, main question, character goal(s), ultimate conflict and resolution.
If you're an established author, you can have a more dangling end, but a new writer has a difficult time getting away with it. The publishing industry is competitive enough. Why shoot yourself in the foot before you jump into the race?
Once you finish your project, work on something else in that same genre or vein. Yes, that same one, just be more terse at your next go. When it's finished and edited, many times over, seek representation for it. Once you sign with an agent, he or she will generally get a first read of your VERY polished duology/trilogy. You'll have a foot in the door plus some respect for your writing ability, and it won't look so much like you have diarrhea of the pen. Perhaps, you can then get a three-book deal. If you show you can write well and entertain the market you're after, it will benefit you greatly.
Every writer should strive to create a satisfactory ending. It doesn't have to be happy. It just has to answer the questions you posed at the onset in the most dynamic way possible. Sometimes there's a twist, tragedy and tears, or blissful kisses. Let your ending resonate. Avoid a dead ending that lands in the grave. End with a bang, something more than what readers expect. The conflict and story arc must lead to the perfect resolution to make readers just a little bit sad that they've reached THE END. Then they'll be salivating for your next book.
~ Signing off and sending out cyber hugs.