In a Project task Finish to Finish relationship scenario, assume there are only 2 tasks in a project. Task B should not be Finished until Task A is finished. Task A takes 2 days and assume it is starting on 5-Jan (also project start date) (for simplicity all 7 days are working days). Task B takes 4 days complete. When i add the dependency of F-F, should the date for Project Start date to be changed to 3-Jan to make Task A and B to end on Same date i.e 6-Jan or should the Project start date to still remain as 5-Jan push the end date of Task B as 8-Jan ?
The finish-to-finish relationship says that you cannot finish B unless A is done. As an example, you cannot deliver a cake which is still not ready.
That being said, B will overlap A (Starting after A and finishing later than A).
Suppose you start your project on January 1st and every single day is a normal working day.
A finishes on January 3rd and B (consequently your project) finishes on January 5th.
Source 2: PMP Exam Prep, Eighth Edition - Rita Mulcahy (Pag. 202)
Both are possible - it depends on the nature of Task B
should the date for Project Start date to be changed to 3-Jan to make Task A and B to end on Same date i.e 6-Jan
In some cases you can start Task B earlier. For example, let us say Task A is 'Add wiring' and Task B is 'Inspect electrical'. You can start Task B earlier by studying the wiring diagram and preparing an inspection checklist. In this case, if you coordinate very tightly, you may be able to finish Task B very close to the time Task A finishes.
or should the Project start date to still remain as 5-Jan push the end date of Task B as 8-Jan ?
In some cases Task B can only be started after Task A starts. For example, let us say Task A is 'Hang drywall' and Task B is 'Paint drywall'. In this case you cannot start Task B before Task A starts. Here you won't have any choice but to push the end date out.
The latter. The project start date is considered a constraint, right? It is a chosen date to have resources on site to begin work and spending money. You cannot arbitrarily start earlier. Therefore, if you have a FF relationship, you need to let schedule push the finish date such that the relationship can be honored.
A good example of this is if you have a management LOE task that has a FF relationship with a discrete task. Management can go no where until the discrete task is finished. Therefore, the discrete task will cause the finish the management LOE task to continue until it finishes.