I guess I did not have any install issues.
Here is the realoem diagram for the midpipe:
Here is the diagram for the headers (nothing interesting here):
I did not have to do any fiddling and did not study the parts that closely. In other BMW exhausts, I have sometimes found that one side is welded in place and the other side has a removable flange and a cone shaped gasket, and the exhaust inserts on one side and butts on the other side.
If you need a new cone shaped gasket, you can buy them online by googling the PN from the realoem diagram. If you need longer flange bolts, you can find those by identifying the threadpitch and then looking for one that is 10mm, 15 mm or even 20 mm longer at a good hardware store or fastenal or similar specialty hardware store.