But it doesn't work today, at least on any Debian-derived distribution. You have to rely on the ia32-libs conglomeration package to have picked up the right version of a library that you want, when it was last updated, which is not a regular occurrence (bugs asking that libraries for Flash 10 are still open 2 years later).
Even if Debian did have an automatic setup for compiling all library packages with both architectures, you are then screwed because they put the amd64 libraries in /lib (with a symlink at /lib64) and the i386 libraries in /lib32. So your proprietary i386 software that tries to dlopen files in /lib fails because they are of the wrong architecture.
You could argue that these are Debian-specific problems. You might be right. But they are roadblocks to greater adoption of Linux on the desktop, and now that the FatELF way out is gone, we're back to the previous situation: waiting for the 'multiarch' fix (think FatELF but with all libraries in /lib/$(arch-triplet)/libfoo.so rather than the code for several architectures in a FatELF-style, single /lib/libfoo.so), which has failed to materialise in the 6 years since I first saw it mentioned. And which still won't fix proprietary software that expects to find its own architecture's files at /lib.