Possibly but this is rare. The other subject will not make up a large part of your teaching load and will typically be lower down the school (KS3). This would only be the case for closely related subjects and only due to the way that schools timetable certain subjects. For example you may be training in History but will have to teach KS3 Geography because schools have Humanities at KS3 rather than separate subjects. If you are teaching science most schools do not separate the sciences before A level and you may be expected to teach all 3 to GCSE.