Keep your characters separate

Each of your characters lives their own life. Don't pull one character into another's storyline, group, or circle of friends — and don't use what one of them knows to help the other. It's a different life, with different people in it.

Not this: your character “Mike” is wrapped up in a storyline, and you bring in “Tony,” who joins the same group, befriends the same people, or acts on what Mike learned.

This: give each character their own friends, their own story, and their own problems — kept fully apart.