When you read a novel, the pace should be smooth, the story should flow, and the character’s head you’re in should be clear. You shouldn’t have to stop and think about which character’s feelings your reading.
The reader sees only what that character sees. Hears what she hears. Smells what she smells. Feels what she feels. Your reader becomes that character for the scene. It can also help you filter out what’s important to that character in that scene.
So, Izzy might think Mel knows something from her expression, but she can’t be sure. Because when you’re in the head of one character, you can’t possibly know what the other character is thinking unless you assess the other’s actions and come to a conclusion, which may or may not be right.
Shifting point of view in the middle of the scene is tricky. It can be jarring, so if you have to do it, try to do it as smoothly as possible. Making the shift clear to the reader is very important. Drop some clues, give the reader the opportunity to realize you’re hopping out of one character’s head and into the other.
And remember, you don’t know what you can’t know…unless you’re psychic.