I have problems with assigning more than one resource to a group of tasks.

The idea is simple, my tasks are in one group and are manually scheduled to particular begin and end dates. 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, the Gantt chart 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 work in the same time, but sometimes MS Project does leveling to do resources work sequentialy.