There are 2 alternatives:
1. Use Minimum Periods
In the Model screen, Increase the minimum periods of, for example, a 5 period block, to 6 periods. Do this by right clicking on the block and choosing Minimum Periods.
In the block screen drag all but one of the classes on to fit in with the sessions 1 to 5. Schedule these 5 sessions.
Then take the other class which has the same teacher allocated and drag it onto session 6.
Go to bands screen and you can see the 5 sessions which have been scheduled.
The 6th session is not scheduled and contains the 2nd class with the same teacher.
On band screen click and drag this class onto the period where the other class has been scheduled. It will then come up with a displace unit window. Place a tick in the lock/clash and this will then lock the clash in place.
2. Place the teacher on the timetable afterwards
After scheduling, you can go to Timetable | New | Department (or another convenient timetable display), right click the class and choose Add Staff – filter for all teachers and find the appropriate teacher, when this teacher is selected, you will be offered the option to ‘double up’.
To room the two classes, use a timetable display. Right click the Room Code and choose Add Room. To add the same room to the second class, use the Add Room function again, search using the radio button on ‘All’ and choose the option to Double Up when the room is selected.