You can install both. The new multiarch support builds blahblah:i386 and blahblah (implicitly blahblah:amd64) debs for source packages that have it enabled. Before that, packages that wanted to provide i386 binaries on amd64 wrote their own extra build support, and added a suffix to the binary package names to avoid collisions.

Here's what I found on how to support LD_PRELOAD without a $LIB dynamic string token:

Looking at this: http://lists.debian.org/debian-devel/2011/12/msg00300.html it seems you could put a relative path in LD_PRELOAD and count on the linker to use architecture-dependent search paths. I'll also link these two threads for context, though they didn't provide answers: http://comments.gmane.org/gmane.comp.lib.glibc.user/974 (chatting) http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=372626 (concatenating the two libs in LD_PRELOAD works, but is noisy).

OK, we got our signals crossed there.