Create a new task, and set the due date to tomorrow. Set the task to repeat every 5 weeks. When tomorrow comes, the new instance of the task (5 week's time) will be created (which is fine). If you then postpone this task once to the day after tomorrow, then when this day comes, another new instance of the task will be created - this time for 10 week's time.

So essentially, within two days of creating the task we now have three occurrences of it. One which is the original, one for 5 week's time, and one for 10 week's time.