You can report on Dual Registered students by going to Reporting > Data Digger.
Select Enrol Status from the first dropdown menu and another menu will appear where you can select the status you wish to report on (e.g. Current Subsidiary: Dual Registration or Current Main: Dual Registered).
Click on the number in the blue circle and it will give you a list of children with that status assigned. That list can be printed or downloaded from that screen.
You could select both options at the same time (see image below) and you will see displayed how many students fall into each category. Here there are 0 students Current Main (dual registered) and 1 student Current Subsidiary (dual registered), seen to the right of the screen.