Do something like:
select DOCNUMBR, DOCDATE from SOP10100
UNION ALL
select DOCNUMBR, DOCDATE from SOP30200
those are just excample fields. but you can only select those fields they have in common i believe. Union the header table with the header table. then a seperate detail with detail. you then join the header to the detail the same as you would before.
I tend to do this stuff in views. So, I (although they can tell you a better way) do the above as one "Header" view then another view of detail, then link them.