Few things will make your story as confusing as a sudden, not marked, shift in perspective. Unless you’re writing from an omniscient view the reader should only be able to glean the thoughts of one character at a time. As a writer this can be hard to do because more often than not there’s another character that has something they could add to the scene if you could just get a flash of what they’re thinking or seeing. If what that other character brings to the scene is so important it needs to be included, there are ways to go about it without throwing readers off.
A break. This is the simplest and nicest way of going about changing character perspective. The only problem with a break is then trying to give that character something longer than the one paragraph you had previously planned. If you don’t think you can, you’ll need a different option. If you do believe you can give that character a decent amount of text, do so. Short breaks are fine, but breaks of only one or two paragraphs are too short normally. You don’t want to jar the readers out of the headspace they’ve got going.
An alternative to a break is to make your main character intuitive. Have them glean clues about the necessary thought from the emotions on the other character’s face. Let your character guess, let them assume. “Judging by Adam’s face I was fairly sure he was thinking…” is a simple example of this. “She/He looked mad,” is another. The emotion from the other character is being displayed, but through the perceptions of the character currently commanding perspective.
You could have your character ask. This is a straight forward method and if something needs to remain hidden from a character it’s not going to work. If it’s not a secret though, this is a quick way of getting the other character’s thoughts out to your readers. “What are you thinking?” “Why are you looking at it like that?” are good examples. This isn’t a method that will always work, but it’s one that should be kept in mind.
Perspective is key to any good story. It needs to be from the character who has the most important things to say at the time, and it should be very clear. One page shouldn’t contain the innermost thoughts of three different characters. Readers will find this off-putting and they will stop reading. If you’re book is being published by a press, you’re editor is going to go through and make you rewrite that entire bit. Save yourself the lost readers and the hassle.