SimplySwim doesn't allow for fortnightly classes.
However, you can create a system to deal with the Fortnightly Swimmers
Firstly, you need to create a new Non Attendance reason. Call it 'Fortnightly Swimmer' and with this Attendance Reason you will mark down there attendance for the student every second week that they aren't attending.
Then create the class as per normal for any other class. You will be able to enrol students into this class normally as well if you decide this class would like to be done Weekly.
When enrolling the Fortnightly Students you need to tick the 'Set Special Price' and enrol the student as half price for the lessons as the Non Attendance reason doesn't give credits for the absence.
Once this is done you can go through and set the attendance of the dates the student/s wont be attending. This then leaves the attendances of the lessons the Student is attending and the other lessons have been dealt with and not have to worry about them.