Are they always the same or not? It looks that they are. But I don't see this clearly spelled in the (ATmega4809) datasheet. Interestingly, one of the older ATtiny417/814/816/817 datasheet says "On this device, CLK_CPU and CLK_PER share the prescaler setting, and hence, always have the same frequency". But for some reason they removed that note from newer datasheets.