The first book revolves mainly around one character. There are several other characters, but 90 percent of the first book is told from the point of view of one character. I stuck with that character's point of view when I started the second book. But, for that character, the second book is really a middle. It gets him from the end of book one to the start of book three. That stuff has to happen to him, but it doesn't have a beginning a middle and an end. The story that does have a beginning a middle and an end is the story of character A's half brother. That's the story I should have been telling.
The two stories interweave considerably. And they come together at the climax. But book two is really character B's book. I'm currently working through, trying to change the focus, but it's slow going. And it's probably going to take several passes to really accomplish just that change, let alone any other revisions. Is this a case where it would be better to just set roughly 400 pages aside and start over?
As for which viewpoint character should be used, I'm of the school-of-thought that it should be the character who is affected most by the events of the story...
Only you can answer that question though. If it helps the first draft of Falcon was really close to 300K words. No, that's not a typo. So I split it into two books. In improving my writing/story telling skills, getting invaluable feedback and editing two things happened. Readers complained about where it ended - ie I was SO disappointed, it was just getting to the really interesting stuff when it ended - and I realized that the first story should have taken a different turn. What did that mean? Most of "Book 2" was useless. It's allowed me to revise Book 1 and rewrite Book 2 so I could complete the story in a reasonable word count.
Look hard at what you've written. Some of it you can probably keep with a POV change and some you're just going to have to throw out and rewrite. I found it to be a pretty fun and liberating process though.
Good luck