When does the second Sprint start?

Sprints are fixed-length events of one month or less that serve as a container for the other Scrum events and activities. Sprints have consistent durations throughout a development effort. A new Sprint starts immediately after the conclusion of the previous Sprint. There is no gap or downtime between Sprints. The second Sprint starts right after the first Sprint, regardless of the customer acceptance testing, the architectural changes, or the Product Backlog items.

The Scrum Guide

Understanding and Applying the Scrum Framework

Professional Scrum Product Owner™ I Certification