Double identity column

I wonder how tricky it would be for the DB team to come up with double 
identity column?

For example,

I have an order header file and I use an identity column for the order 
number.  I insert a new row and use Identity_Val_Local() to retrieve the 
value of this new order.  Now I insert into the order line file.  However 
I have a known order number, Identity_Val_Local().  Now I want it to 
increment the order line number as a identity column.  However I want it 
to start over at 1 for each new Order.  Like the following:
Order     Order Line
00001     001
00001     002
00001     003
00002     001
00003     001
00003     002
00004     001

Rob Berendt
-- 
"They that can give up essential liberty to obtain a little temporary 
safety deserve neither liberty nor safety." 
Benjamin Franklin