The order of users selection cannot be stored and hence your request is not possible.
I understand that there may not be a direct solution - but a roundabout way may be possible.
I tried a calculated field with NOW() which applies the current system time each time a selection is made. My issue that this calculation is recalculated for each record upon each selection. Surely there is some way to get this to work... Does Tableau have a temporary method of writing data to a record so I can store the original timestamp, or original index calculated, once a selection has been made?
I will try to look into it over the weekend.