I don't usually use range parameters for this specific reason. I'll add two parameters - one for start date, one for end date and set the filter in the Select Expert to date field between {?StartDate} and {?EndDate}.
However, if range parameters are set up like multi-value parameters, you should be able to get both values from a range parameter by treating the parameter as an array of two values and getting the value from each of the items in the array.
-Dell
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