There's some synchronization effects in metronomes and in industrial equipment. If you have several things that can oscillate, perhaps not at precisely the same rate but close, and the motions reinforce each other when they do achieve the same rate, you can get surprising synchronization.

For an intuitive example, see this Veritasium video, starting at around 4:50: https://youtu.be/t-_VPRCtiUg?t=292.

This happens in crowds of people, for an example, the London Millenium pedestrian bridge wobble: https://en.wikipedia.org/wiki/Millennium_Bridge,_London#Reso....

I was initially curious how this could happen with a heart rate; I could imagine some shared feedback mechanism for walking, or breathing, but a shared feedback for heart rate was surprising. Perhaps if they were touching, and there was some electrical or pressure-sensitive pulse? Disappointing that it's merely the delta between exciting and dull parts of the stories.