Also I do not agree with the premise that we can only think about one thing at a time.

We routinely communicate with multiple people at once and also communicate with the same persons in multiple threads of conversations.

Of cause this means that we switch between those tasks and do not really do them in parallel. At most we listen to one person, answer a second via speech, a third via text while thinking about what to respond to a fourth

We just switch our focus of attention quite fast