I've been toying with apt-build, not in small part because I got a Haswell processor, and some of the software can run much faster if compiled with the right flags

Anyways, I am stumped because I have packages like this:

ii  libgstreamer0.10-0:amd64               0.10.36-1ubuntu2                                                       amd64        Core GStreamer libraries and elements
ii  libgstreamer0.10-0:i386                0.10.36-1ubuntu2                                                       i386         Core GStreamer libraries and elements
How do I get to produce those multi-arch packages? Any ideas? My build only produces amd64 packages, so I can't really upgrade to the packages I build (and there is always the odd piece of software relying on the 386 packages)

Many thanks for any pointers! I googled, I man'ed, and no charm