Can a teacher who is registered in Secondary subject A teach in subject B?

No, the teacher requires to be registered in subject B.  Professional (Additional) Registration is a way for teachers who hold Full Registration in a Secondary education subject to add additional subjects to their registration.

Teachers require to be qualified in the secondary subjects they teach. There are routes available for fully registered teachers to achieve provisional registration in an additional subject where they hold the academic requirements and provisional conditional registration where they hold some of the academic requirements. These routes should be explored in supporting teachers to gain further categories of registration. Teachers being qualified in the subjects they teach is central to trusted teaching in the public interest.

Learn more about Professional (Additional) registration.