The Agile community talks about this a lot, but I have actually only seen this in two context2: (1) people being pulled from a task to work on a production support issue., and, (2) people being on more than one team at a time.

Aside from these two cases, in my 45 years in software development and 20+ years of experience with countless Agile teams, I do not recall every seeing someone being given more than one story to work on at a time on a given team.