I don't believe that setting "Multi-Arch: same" would be sufficient, as both packages install files to /usr/lib/gtk-2.0/2.10.0/engines, so one would probably overwrite another (which is not what we want).
What would probably need to happen is for both packages to install their libraries to /usr/lib/<arch>/gtk-2.0/2.10.0/engines. Moving the libraries there manually WorksForMe(tm). So, Control file: Multi-Arch: same File paths: (386) /usr/lib/gtk-2.0/2.10.0/engines/libxfce.so -> /usr/lib/i386-linux-gnu/gtk-2.0/2.10.0/engines/libxfce.so (amd64) /usr/lib/gtk-2.0/2.10.0/engines/libxfce.so -> /usr/lib/x86_64-linux-gnu/gtk-2.0/2.10.0/engines/libxfce.so -- "Nothing unreal exists." - Kiri-kin-tha's First Law of Metaphysics.
