[jvdongen]

If, as the article implies, the same user gets the same identifier across all KISSmetrics customers they ARE (implicitly) sharing customer activity between customers.

As the article states, customer A and B could simple put the information they each have about identity XYZ together. Without KISSmetrics having to be involved any further than in providing and maintaining the unique identity to each customer separately.

Even if it was not the intent of KISSmetrics for this to be possible (which I've a hard time believing), the chosen implementation makes it possible.