I strongly suggest that you abandon the duplicate app concept. It will cause you no end of grief. For starters you will have Duplicate maintenance and an inability to get consolidated Dashboards or even a simple consolidated report of Tasks. Every user will need to go to two places for no good reason. Twice the number of report subscription emails of My Tasks. The list goes on and on.
I suggest instead that you make a required multiple choice field checkbox field to indicate if the task duration is in days or business days. Then have a formula use that information to calculate the correct due dates.
