How to assing resources in MS Project 2010 to keep begin, end and duration of a task intact?

Question

I have problems with assigning more than one resource to group of tasks. Idea is simple: My tasks are in the one group and are manually scheduled to particular begin and end date. I want to assign more than one resource to keep task duration and dates (fixed duration) and increase work.

For top level tasks it works fine but as long tasks are grouped, the duration of each is extended to reach group end date and work remains...

For the problematic tasks, Gannt looks like this:

One resource attached (good)

 (  Task 1.1 )
              ( Task 1.2  )
                           (Task 1.3)

More than one resource attached (wrong)

 (  Task 1.1 ).......................
              ( Task 1.2  )..........
                           (Task 1.3)

So for tasks like that i want to have a fixed schedule and just increase work by adding resources that works IN THE SAME TIME but sometimes MS Project does leveling do resources work sequentialy.

Answer 1

At least in MS Project 2010, when adding a new resource to a task from the main, table interface, Project will ask you how to deal with that new resource: