You don't actually need two different time pickers to do this. One will allow you to select any slice of time imaginable. Basically, you would just edit the dashboard, add a time picker, then for each element in the dashboard ensure that the time used for the search will be the time chosen with the picker instead of the existing time selection for the search. Here's the doc link: http://docs.splunk.com/Documentation/Splunk/6.1.3/Viz/FormEditor#Add_a_time_input_to_a_form . Hope that helps.
... View more