internal tables in WS_download

HI

i have defined 2 internal tables one for invoice header details

and another for line item details.(both have different details) now i have to download this data to text file in this format

invno1 text....

line1 text...

line2 text..

...

invno2 text..

line1....

line2...

how to do this? if i use ws_download for 2 tables seperately i am getting following o/p.

invno1 text...

invno2 text....

line1 text...

line2 text....

line1 text....

line2 text...

any one plz help me...