My book is mainly focused on one character, and is written in first-person, until about half-way through. In the middle of the novel, the perspective will shift into third-person and focus on a different character the readers haven't heard of before. Their two stories will eventually come together towards the end of the novel and resolve. But I was wondering if this is to confusing for the reader to have the book change perspective and character focus half-way through.
You are free to change a story's POV through the story. You can see this technique used in many novels that contain multiple plots and subplots. Dune, Game of Thrones, Count of Monte Cristo, and many more novels to name change the POV in between chapters.
It is an effective technique to build tension and sustain suspense since it can lead to engaging cliff hangers and evoke the what is going to happen next instinct in a reader.
Less common is changing between 1st person POV and third person or 2nd person or whatever. But, as long as you are consistent with which POV you show a character, it's an accepted technique.
Structuring your story so it focuses on one character's POV for the first half a story and then changes for most of the rest of story to another character's POV has the potential to throw away the stories momentum and reader engagement with the character they'd been following. This is why I think multiple POV novels are written with alternating chapters so the reader introduced to all the important character's early and wants to know what happens to all of them. In that the shifting POV becomes a source of tension in the story.
That is not to say your idea won't work, otherwise. If the two characters are separated by a long period of time, and your novel is telling about two periods of history that are connected somehow. Then it might be very necessary to tell the story that way.
Or, if the alternative 3rd person POV character whose POV we follow later in the story, is actually included as part of the 1st person's POV before the narration changes to their POV in the 3rd person, then that might help sustain engagement at the point the narration changes POV.