When people cord to one another, does a single cord contain all the memories representing key events in the relationship, or are there multiple cordings, one for each key karmic event?

There are, indeed, multiple cordings, each representing a significant karmic interchange. Keep in mind, these are things that are seen by the Law of Karma as having significance, good or bad. So not every interchange will create a cording that lasts. There are many launched that will fade because they are of minor importance and they will naturally be trimmed away in a housekeeping roundup done at regular intervals. But each cording is for a specific reason and purpose and has a specific origin. When things are quite positive, there will be many positive energetic cordings, and when there is a tempestuous relationship there may be quite a large family of cordings with a negative energetic origin and character. Even if they are all quite similar, they will be discreet because the system is designed to be precise and does not presuppose a pooling of things for the sake of simplicity or efficiency.