Many documents into one spool request

Question

How can I control the opening & closing of spool requests in ABAP?
The context is a program that prints dunning letters and their associated billing documents.
We want the hard copies to appear in the correct order (dunnning letter, its billing documents, dunnning letter, its billing documents,...)
One way to achieve this could be to have a single spool request for each "dunning letter + billing documents" set of documents.
Currently the documents go into the spool-request list in the correct order (in separate spool requests), but that does not ensure the correct order for the hard copies, since we have more than one work process handling the spool queue.

Answer 1

My program is working: for each dunning letter and its billing documents there is one spool request.  Thanks, Christian.
Some key points...
[1] Invoices in table MHND can be other than SD invoices.  The non-SD ones were disregarded - trying to print them as SD billing documents would fail, and maybe mess up the logic for opening/closing spool requests.
[2] For each billing document, I used itcpo values exactly as for its dunning letter, except that:
for the dunning letter, itcpo-tdnewid = 'X' and itcpo-tdfinal = ' ';
for billing documents other than the last, itcpo-tdnewid = ' ' and itcpo-tdfinal = ' '.
for the last billing document for the dunning letter, itcpo-tdnewid = ' ' and itcpo-tdfinal = 'X';
If you use different values of any of tddest, tdreceiver, tddataset, tdsuffix1, tdsuffix2,..., for instance, then you will get a new spool request, whatever the values of tdnewid or tdfinal.
[3] Fortunately we already use our own print program for billing documents.  So just before the CALL FUNCTION 'OPEN_FORM', I tested the value of sy-cprog, and if it was the name of my report then I assigned as in [2] to the structure for the OPTIONS (i.e. itcpo) parameter of OPEN_FORM.
[4] To pass values for [3] from the report to the print program, I used export/import to/from memory.
[5] Because I was controlling the itcpo values, I was able to arrange that something helpful to the user appears in the Title column in 'Own spool requests' (transaction SP02).  This title comes from the itcpo fields tddataset, tdsuffix1 & tdsuffix2, and I arranged that the dunning level and the customer number appears in Title.