I'm going to describe as best I can, but you haven't provided a packaged workbook, so I'm making assumptions.... I think you'll want to do something along the lines of :
1) Create a field that stores the Latest Visit Date : WINDOW_MAX(MAX([Visit_Date]))
2) create a field that stores the no of devices if Visit Date = Max Date from filter, something like : Count Devices on Latest Visit : IF ATTR([Visit Date]) = [Latest Visit Date] THEN SUM([No of Devices]) END. THis will provide a field like your 'No of Devices' column but ony have values populated for the rows where the date matches.
3) Create another field that sums this column across all the rows in the table: Total Devices on Latest Visit : WINDOW_SUM([Count Devices on Latest Visit]).
These use table calculations, so you may need to alter the properties of the calculation to ensure the values are being calculated over the appropriate partitions (dimensions). If these aren't familiar to you, please refer to Transform Values with Table Calculations - Tableau .
Attached is a similar concept using Superstore Data, in v10.1, and uses Order Date and Qty instead of your Visit Date & Device Count.
Hope that helps, please feedback.
ForumQ.twbx 396.6 KB