How to launch an app immediately after another app is closed using Tasker
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).