Attached is a patch to atkmm1.6 which transitions it to use multiarch library paths.
Since libatkmm-1.6-dev contains no build-time-generated include files or arch-dependent scripts/executables, it can be safely marked as Multi-Arch: same, so this patch does so. Because many of its dependencies (at least libatk1.0-dev and libglib2.0-dev) have not yet been transitioned for multiarch, libatkmm-1.6-dev will not currently be multiarch co-installable. This is fine, though, and if all of the dependencies were ever to be transitioned for multiarch, the libatkmm -dev package would immediately become multiarch co-installable.
Typically the biggest outstanding blocker to multiarch transitions is .la files with non-empty dependency_libs lines. However, I checked all files listed in http://lintian.ubuntuwire.org/tags/non-empty-dependency_libs-in-la-file.html (http://paste.ubuntu.com/766579/) and atkmm isn't listed anywhere, so this transition should be safe now by that metric.
Thanks for your work on this. I took the liberty to update debian/compat to 9 - AFAIU http://