Are you using resources in your project? The big picture I suggest is this:

Create two calendars (Tools, Change Working Time) for each 12 hour shift.
In Tools > Options, Schedule set hours per day to 12 if you want to use the shift as a measure, hours per week to 84.
Assign the shift calendars to the resources (set their base calendar in the Resource sheet).
As you assign resources to the tasks, the task will shift to accommodate the shift for that resource.