Hi,
That smartform does not permit to simply include another smartform is a limitation, but I don't believe there a solution for combining smartforms that would sufficiently communicate developer's intent, and would be simple and transparent enough...
It would be possible to get the output of both smartforms as OTF data and then combine them. Renumbering pages across two OTFs is even fairly easy, provided the driver program (or something else - let's call it "output control module") keeps track of the last page number (in the spool). Algorithm to strip away OTF commands that indicate end of one form and OTF stream and the start of next OTF stream and the form would also be possible. Resetting the tray selection and print mode for first Open Page command of second form would also be possible. OTF format description can be found here. I guess it would be a fairly robust solution (and I don't think the OTF format itself is being extended anymore).
OTF manipulation and writing to SAP spool is can even be made into separate "output control" module with an API, which smartform driver programs or Print Workbench Forms then can use. The SAP Print Workbench Application Form type Collection permits to combine Several Application Forms (Smartform or Sapscript based) and thus kind of express intent - I'm combining these forms into one "Document".
I have combined several Application Forms via renumbering the pages in OTF, so that the Output Management System, which receives the resulting Postscript (produced by SAP Spool System) recognizes both forms as one "Document" and creates one PDF out of them it. Do I think at the end of the day that it's an elegant, transparent, simple to maintain solution? Absolutely no - like every "proprietary" solution of that kind. Would I do it again if I had to..? Yes 
cheers,
Janis