If I buy the Platinum pass at Kings Island, and the dining plan, can I use the dining plan at Cedar Point when we visit there? 

If you buy the platinum pass dining plan then yes you could use it when you visit Cedar Point or any other Cedar Fair Park.

1 hour ago, Rllhenry said:

If I buy the Platinum pass at Kings Island, and the dining plan, can I use the dining plan at Cedar Point when we visit there? 

I think you have to make sure you select the Platinum Dining plan option. 

I don't think you can buy gold dining with a platinum pass.

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Correct you have to get platinum dining but it is only like $25-$30 more depending on when you upgrade.  As long as you eat two meals at the other park it pays for itself.

But kept in mind the meal plan will not be valid until 2020, but the pass can get you into the park in 2019....