How to launch an app immediately after another app is closed using Tasker I need help creating a Tasker profile that will open Aldiko Sync  ONLY after I have ended the main Aldiko Premium app.

So with this profile, Aldiko Sync automatically launches and does its thing immediately after I close Aldiko Premium reader.
You can setup an exit task. An exit task is assigned from the Profiles tab. Expand the entry of your profile, long-press the assigned task there and choose Add Exit Task.
(Click image to enlarge)

Instructions:

Profile: Application → your app (Aldiko Premium in this case)
Task: (Actions): do anything you want
Exit task: (Actions): App → your another app (Aldiko Sync in this case)

Note: It is not mandatory to have an enter task in order to assign an exit task to a profile.
If you don't want an enter task i.e. don't want a task to be run when the profile is triggered, you can choose to create a task (supposed to be used as an exit task) under Tasks tab, assign it as an enter task to a profile and make it an exit task using Move to Exit (see screenshot).