The current FAQ says you do not need to repeat the written exam, provided you retake the performance exam within one year. Take longer than a year and the written test must be repeated as well.

That makes the one-year window the thing to plan around. Once the written result is in hand, book the performance retake inside the year and the written paper stays valid. Let the year pass and you sit both exams again.

The rule cuts both ways, which is worth remembering when the results arrive in the other order: if the written test is the one you failed and the performance exam is the one you passed, the written test has to be retaken on another date within one year.

FAQ answers are policy, not physics, and policy gets revised. Check the current version again before you book, and do not rely on a cached page or a forum post. The date of the retake matters more than the memory of the rule.

Source

From the CI preparation and examination FAQ; the same answers are reproduced on the CI exam FAQ; verify the current version before booking.