One overview or two when the task has two charts?

One. The overview is a single statement about what the visual information shows as a whole, and the descriptors ask for a clear overview rather than one impression per image. Two overview sentences usually mean you have written two descriptions and put a summary in front of each. If the two visuals genuinely share nothing measurable, say so in one sentence and move on; that sentence is still the overview.

IELTS Writing Task 1: When the Task Gives You Two Charts →