Compare two values from same filter and identify one field as original value

I have a filter with multi-select drop-down where I only want the user to select two values only on the list. I also need user to select which value should be used a original value so that this formula can be applied

(Original value - Compared to value)/SUM(Original value)

If I use two parameters to control the filter, only one parameter works at a time. So if I can determine which value was clicked on first, would like to set it to Original value, or use a calculated field to indicator if one of the values in the drop-down filter is Original value or not.