The Second Brain template is more of a personal productivity and knowledge management system.
On the other hand, Business OS is a more comprehensive template when compared to Second Brain, as it comes with features for managing content creation, products and services, and CRM as examples of having views and databases more suited for businesses and organizations. It is also built to be more for teams or multiple users, where you can assign people to tasks and create a dedicated dashboard page for each member.
You can use both templates separately, where Second Brain is for personal use and Business OS is for your business.