Acredito que você precisa ter um segundo pacote que seja Architecture: i386
-only e Multi-arch: foreign
:
Package: mybuildsystem
Architecture: amd64
Depends: mybuildsystem-i386, <amd64 stuff>
Package: mybuild-system-i386
Architecture: i386
Depends: libncurses5-dev
Multi-Arch: foreign
Porque 1 :
Multi-Arch: foreign
The package is not co-installable with itself, but should be allowed to satisfy the dependencies of a package of a different arch from itself.