The current means for providing AMD64 32-bit support within a 64-bit Linux environment is a BAD thing.
Split directories, one set for 32-bit (/lib, etc.) and one set for 64-bit (/lib64, etc.). It confuses the hell trying to make a cross-compile setup. It leaves it open for you to miss libraries on either side, causing you to not be able to do the stated thing, run 64-bit binaries alongside 32-bit ones. And it confuses the hell out of each and every automake setup.
We all need to really, REALLY re-think this idea. There's a need/desire for someone to run 32-bit applications- but this isn't the way to do this. Do I have answers for this? Not yet- I've not had the time to think about it seriously.