Per my test, it is not possible to create a refinable managed property that combine the values of both fields to generate as a date & time field.
In the SharePoint search, you could use refinable managed property equal to a value to filter search results. It can be a value from one filed but cannot a combined value form two fields.
So, as a workaround, you could use two refinable managed properties to filter search results or create a date and time field as refinable managed property to filter search results.
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