From a fresh sid chroot:

Install multiarch glibc

install build-deps

get the source

look at debian/rules

put dh_link and dh_installdirs above dh_installchangelogs and dh_installdocs

Add to the configure line:
 --includedir=/usr/$(DEB_BUILD_GNU_TYPE)/include --libdir=/usr/$(DEB_BUILD_GNU_TYPE)/lib

Add -common packages to control, the old lib packages should be depending
on the -common package.

Splitting the packages:
create libpackage.links with the line:
usr/share/doc/libpackage-common usr/share/doc/libpackage

Move common files (examples, docs, m4, etc) to -common.{examples,install,...}

And on the files containing those paths (lib* ?): 
perl -pi -e 's;^debian/tmp/usr/include;debian/tmp/usr/i386-linux/include;'
perl -pi -e 's;^debian/tmp/usr/lib;debian/tmp/usr/i386-linux/lib;'