Some packages I've created here have extensive documentation, which logically falls into the "noarch" cathegory. I've been unable to create package.<arch>.rpm and package-docs.noarch.rpm out of the same spec file. Possibly just because I'm dumb ;-)
All sub-packages of a single spec file must be in the same arch (i.e. only one arch is permitted during a single spec file parse and build). The easiest fix is to split the doco into a separate package. More complicated is to write a meta spec file that does two builds with two included spec files.
Perhaps RPM needs another keyword to indicate that a package (or a subpackage) is architecture-independent. The current situation is that building a noarch package is building for another architecture. Building for i386 and noarch on i386 is for RPM something like building for i386 and alpha on i386, i.e. compilation for two architectures in a single build. That's why RPM cannot do it. But this should be fixed. There are many examples in RedHat-7.0 where separate source packages were created to work around this bug, e.g.: XFree86-KOI8-R-1.0-2.src.rpm gimp-data-extras-1.1.20-3.src.rpm sgml-common-0.1-10.src.rpm
I just ran into the same problem. I think this is highly undesirable. In fact, this doesn't just apply to the noarch architecture -- it should at least be theoretically possible to have a package that produces multiple architectures from the same source.