defining mass allocation

Hi  All

I am defining mass allocation to my client

There are 5 segments in my flex field

 I want to allocated my total expense to department (there are 2 department) then again allocated that to branch (there are 12 branch)

My client will give us the standard for this allocation, from that we can define stat journal,

According to my understand there should be a two allocation

1)      For  total expense allocation to department

2)      Then that total expense allocated to department  allocate again to branch

For that we have to pass stat journal with allocation ratio  

But problem is how we can allocate this parent level amount like expense parent for first allocation and department parent for second allocation. Because in the journal I can’t post stat journal