Courses that are cross-listed have separate groups by default. This allows instructors to create a single activity and choose whether they want each section of participants to view content and interact with the other.
Instructors can also create groups and divide activities based on their choosing. However, more often than not, you'll probably want to turn off Groups to allow students to interact between the cross-listed sections of your course.
To turn off groups:
For more information about groups, you can read Moodle's Support documentation https://docs.moodle.org/35/en/Groups