I found the answer.
I needed a p2.inf file sitting next to my feature.xml file in the first feature I mentioned in the question.
And in that p2.inf file, I needed this text...
requires.1.namespace=org.eclipse.equinox.p2.iu
requires.1.name=com.myplatform.specfic.bundle.win32.x86
requires.1.filter=(&(osgi.os=win32) (osgi.arch=x86))
requires.1.namespace=org.eclipse.equinox.p2.iu
requires.1.name=com.myplatform.specfic.bundle.win32.x86_64
requires.1.filter=(&(osgi.os=win32) (osgi.arch=x86_64))
I added this file to the build.properties as well to make sure it was included in the deployed feature
Then when I deployed the p2 repository and performed the install into my RCP application, the correct plugin was placed onto my 64 bit machine.
UPDATE: I was wrong here. This is NOT the correct answer. The incorrect plugin still gets installed on some machines.