iTunes maintains a couple of files on your laptop that tell it where to find anything you have added to the library. Problem is you haven't added your external drive yet.
Laptop B iTunes does not see the files on the external because you have not told it they exist. It doesn't matter that you pointed B to the drive in advanced preferences, that only tells iTunes where to put any files you rip in the future, it does not tell iTunes to add anything already on that drive to the library. You can either go to the menus and add to the library or drag the external music folder into iTunes and drop it. iTunes will then add the song titles you selected to it's library files along with the path to get there and it will know where they are.
Sharing will only work if the 2 laptops are networked together. If they are then the iTunes library from A will show up in B's library as shared. This isn't really what you want since A will always have to be on and the external drive hooked to A. That means B will be pulling the files through A and the network rather than directly from the drive.
Laptop B iTunes does not see the files on the external because you have not told it they exist. It doesn't matter that you pointed B to the drive in advanced preferences, that only tells iTunes where to put any files you rip in the future, it does not tell iTunes to add anything already on that drive to the library. You can either go to the menus and add to the library or drag the external music folder into iTunes and drop it. iTunes will then add the song titles you selected to it's library files along with the path to get there and it will know where they are.
Sharing will only work if the 2 laptops are networked together. If they are then the iTunes library from A will show up in B's library as shared. This isn't really what you want since A will always have to be on and the external drive hooked to A. That means B will be pulling the files through A and the network rather than directly from the drive.