debi, debc: list other-arch packages, install other-arch M-A: same packages
debc: List any architecture's packages
If you have a merged changes file for amd64 and i386 or similar, it seems reasonable to list the contents of any package, even if it would not be installable on the current system (similar to the way we list udebs even though only d-i can install those).
debi: Consider installing foreign 'Multi-Arch: same' packages
Multi-Arch: same packages need to be installed in lockstep. For example,
if you have a merged .changes file for dbus on two or more architectures,
and you use
debi --upgrade dbus_x.y-z_source+amd64+i386+all.changes
while libdbus-1-3:amd64 and libdbus-1-3:i386 are both installed, it seems better to install both than to fail to configure some packages. However, installing dbus:i386 is undesirable, assuming dbus:amd64 is the one currently installed.
The logic used when dpkg -Oi decides whether a package is already
installed appears to be: if it is Multi-Arch: same, see whether this (package, architecture) pair is already installed; if not, see whether this package is already installed for any architecture.