You will first need to set an enrol status for the student. Go to the students profile and click on the "Extended" tab.
Go to the School information section and find "Enrol status" - click the drop down box. This will give you the option to select if you are the Current Main (dual registration) or Current Subsidiary (dual registration) school.
One you've made your choice, scroll to the bottom of the page and click "save" to make those changes.
You also need to set up a register pattern for the days that the child is not at your school with a Dual Registered code. This can be done via Register> Patterns
Guidance on how to set up patterns can be found Here