I have seen threads where people are installing one or both versions, but I am not seeing the option.
I am experiencing the same issue I did with X3 64bit with bridged plug-ins. Everything loads and executes fine, but then there is the inevitable crash on save. Not fun.
However, if I run X3 32bit, the problem does not show up. I'm ok with 32 bit. It's stable, and I only have 4Gb RAM anyway.
So how do I get to the 32 bit version of Sonar Professional?
For what it's worth, the 64bit install was silky smooth, and even the challenge/response went through quietly with no intervention from myself. Bear in mind, it was installed on a machine with previous Cakewalk products.
Not complaining, just looking for an answer.

When running on a 64bit OS, you will need to run the installers by hand to get the 32/64bit prompt in order to install a 32bit version of SONAR. C3 caches the installers. The installers are stored by default in C:\ProgramData\Cakewalk\Command Center\Downloads, the actual location is set in the C3 config screen.

Cool. Got it. Thanks,scook.
The 64 bit versions of X3 are still crashing on exit/save, so I  need to send a crash dump to Cakewalk, but I'm ok with 32 bit.