No, that didn't work either. It looks like scook is right and this is no longer possible.
 
HOWEVER, I found a better solution - a bit of a jury-rig but it works, and for the most part better than the idea of using 32-bit and 64-bit versions simultaneously: I host Komplete Kontrol in Kore, and then the newer VST versions in Komplete Kontrol. This allows mapping Kore's lovely knobs to control things in newer Reaktor ensembles and so on. Cool!

If you rename one of the plug-ins, I think the CLSID will be different and hence SONAR won't pick up that they're the same.
 
Worth a try anyway.

Tried that too - nope. Sonar is too clever for such tricks! Renaming triggers a re-scan, but when it does so it identifies the plugin and hides the 32-bit one if the 64-bit one is present.
 
Anyway as per previous, no big deal, probably better this way as there's a better solution for my particular situation, and otherwise it's hard to think of a valid need for both to be available...