On an x86_64 system, you can install i686 packages. But then you have already base x86_64 userland packages installed, especially those with shared libs. Additional package installations will cause conflicts with those base-installed packages.
On an i686 installation, no x86_64 kernel package is available, like it is in Debian. "yum list 'kernel*'" tells so.
Of course, you can always create your own completely-self-made-up package list and use Anaconda or AutoYast or such. But this is what I called "roll-your-own installation" above, then you create a spin. It is not an installation mode that is supported by a distribution out-of-the box -- which is what I was asking for.
I don't deny that it ain't possible; after all, one can also always roll one's own kernel and install that. I hinted to the fact that such an installation mode is not widespread because it is *not a mode commonly offered by distributions*.