How to divide a single task into several different days? 

Let me explain, John has a 24-hour task. I schedule it for him Monday through Thursday.

He has another task from another project at 8am on Tuesday that I can't move.

Currently, in order to have a fair resource management, I divide the 24h task into 3 tasks of 8 hours that I place Monday, Wednesday and Thursday. This becomes complicated to manage with about 20 divided tasks that I have to shift continuously. Do you have a solution?