While I have not read your book and therefore cannot comment on it specifically, I would like to offer my general opinion on splitting one book into two. I enjoy stand alone books. I do not mind books that are part of a series, but I do have a stipulation that is a huge factor in whether I want to read the next book in the series. No matter how great the first book is if the book ends on a cliffhanger and has no resolution, I do not want to read the next book.
Whether the author cares enough about his/her readers to give them some sort of resolution to hold onto while waiting to read the next book is a big element that helps me decide to read the rest of the story. If the first book doesn't actually end there is no reason for me to want to read the next book, I am just upset with the author for leaving the book as a cliffhanger. I don't mind if the story isn't complete, but there has to be some resolution some conclusion for the book to stop where it does. While the story might not be over, I might not be able to get to the next book right away for whatever reason. Give me something concrete to hold onto while I anticipate the second book.
If this can't happen with the book you have written, or are willing to change, my advice is to leave it as is. In my opinion, the only other reason to change the length is if the material that is making the book so long isn't actually needed in the eyes of the reader, not just in the eyes of the author.