Help needed for splitting a file

HI,

i am getting multiple bill related data in single file.. the structure looks like below

Bill_list 1:1

-- Bill_data 1:unbounded

now i want to split the same file into single bills each.

for example say i am getting data for 2 bills in source file.then i want my output to be 2 files like

Bill_list

--Bill_data(1)

and

Bill_list

--Bill_data(2)

I tried using multimapping by changing target(same source file structure) occurance to o:unbound

and mapped

Bill_data-----> Bill_list (o:unbound)

But my file is not getting splitted into 2.. it says 2nd file cant be created.. Can you suggest where i am going wrong.

TIA