since you don't give us some sample code we cannot provide you exact snippets. There are plenty ways to fill a tableView so how should we know how you did that?
Nonetheless I will tell you how it should work abstractly. You will need either an array with the current selection or - as @ Ricard Pérez del Campo suggested - you add a property to your data-model which contains the state (e.g. selected).
Step 2 you will check this property (or array) every time you
a) change the selection OR
b) the tableView is displayed on screen (viewWillAppear and popup appear)
Therefore you need a dataSource which is available from all places (in your case the Popover, the splitView and a detailView)
You said one tableView already work so the problem for the second is probably the selection dataSource.
There is also a method in UITableView called indexPathsForSelectedRows which will give you an array for the current selection. You could apply that selection to the other tableView but it's a hackish way and you should definitely change your data-model instead.