1 Website or 2 websites for adsense, whats prefer?

Hi, i have a website with informtion about parking reports in every city in the country,
after i want to expand the website for 1 more topic, some thing like information about paying water in every city in the country. now, my question is what is prefer, to create new website that only talk about paying water or make it in 1 website? i heard somewhere that when that traffic in the website is grow, then the cpc is going down..

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