By creating a feedback loop of emotion. Character A feels anxious, which bleeds to Character B through the link, making B anxious, which amplifies A's anxiety further, and so on. The plot can use this to spiral a simple disagreement into a catastrophic psychic meltdown where they can't tell whose feelings are whose. The tension is in breaking the cycle. They have to learn emotional regulation not just for themselves, but as a unit. It makes their emotional maturity a shared, active project, which is a great source of ongoing low-grade tension and growth.