You have different options:
- Often, when a novel is told from two viewpoints, the distribution of viewpoints is systematic. For example, all even chapters are told from one viewpiont, all odd chapters from the other. Or one part of the book is from one viewpoint, the next part from the other.
If that is how you divide viewpoints, simply use the viewpoint whose turn it would be.
- Depending on what your story is about, what its theme is, how it ends, and so on, you should have a gut level feeling of whose viewpoint
- drives the plot forward in that chapter
- doesn't give away the plot-driving riddle
- provides more information or entertainment for the reader
and so on. Depending on where you want the focus, chose the viewpoint of the character who
- undergoes a change in the chapter
- is the agent of change
- has relevant knowledge to reflect on and make sense of what happens
- discovers relevant knowledge
- experiences more intense emotions
- lies to the other character (if you want the reader to know)
- is lied to (if you want to deceive the reader)
- provides the female viewpoint (if yours is a feminist novel)
- etc.
- You can head-hop, that is, mix viewpoints from paragraph to paragraph, or even within the same sentence.
Stephen King is famous for this, and even Hemingway did it.