Installing 32-bit Sonar after 64-bit?

It'll be fine. I currently have four versions of Sonar on the same machine: 64 and 32 bit versions of both 8.5 and X1.

The thing to note is that you don't install anything "on top" of anything else. For one thing, Windows won't let you do it that way, and will put 32 bit programs in a different Program Files folder to 64 bit programs.

You'll have to install the patches again for your 32 bit version too.

just accept all the defaults and you'll be okay. It's uninstalling that usually causes trouble.

Depending on how many 3rd party plugins you might have and how co-operative they are you may find the following useful. If it doesn't apply to you feel free to ignore it or use only any bits useful to you.
 
Having both the 32 and 64 bit versions of SONAR on the same computer doesn't of itself cause any problems. Many of us run multiple version in both 32 and 64 bit without problem.
Problems can however occur when you tell SONAR to scan for VST plugins. when running 64 bit SONAR there may be 32 bit plugins that you want to run because a 64 bit version doesn't exist yet. That means having to scan the location where those 32 bit plugins are as well.
That's not a problem either except that when using 32 and 64 bit versions of SONAR you may want both the 32 bit and 64 bit version of particular plugin so each program can make use of it. The rub is that some plugins don't behave and you end up with both the 32 bit and 64 bit version being scanned by SONAR's VST Scanner and you have 2 of that plugin appear. 
 
I have an additional folder just for 32 bit (x86) plugins for those where the manufacturer wants to always install both a 32 and 64 bit version. When it works properly then the 64 bit VST scanner doesn't load the 32 bit version but when it doesn't work properly it means I end up having 2 of the same plugin where the 32 bit one is also running through  Bitbridge. For those plugins only I use the 3rd folder. Working out which plugins don't cooperate has to be worked out by trial and error.
 
To get around all that I have my system setup as follows.
1. Program files\Cakewalk\Vst Plugins\Manufacturer\plugin
All 64 bit Plugins
2. Program files (x86)\Cakewalk\Vst Plugins\Manufacturer\plugin
All 32 bit plugins you want to run in x64 because no native 64bit version exists.
3. Program files (x86)\Cakewalk\VST Plugins(x86)\ Manufacturer\plugin
All 32 bit plugins for which you already have 64 bit versions and the VST scanner insists on installing the 32 bit version as well under bitbridge giving you 2 copies of the same plugin in SONAR's Plugin manager.
SONAR x64 scans 1 & 2
SONAR x86 scans 2 & 3
Apart from folder 3 the others are all default locations if you allow SONAR to just perform it's normal installation.

As far as I understand it, the system doesn't understand that 32 bit and 64 bit versions of synths are the same thing.

Change your VST settings in the 64 bit version to scan your 32 bit plugin folders as well as the 64 bit ones. This will result in you having some doubled up plug-ins, and you#'ll probably want to check through them and re-name some of them so you know which version is which. Bit of a faff, but worth it.

Odd.

Actually, I seem to recall a lot of people reporting a bit of pain getting their DimPro installations up and running. It was okay for me, but there were definitively some issues. Maybe someone will pop up who knows about this, but if you have a search for DimPro installation threads, you'll get plenty of results I'm sure.

I remember when X1 came out there were a couple of demo tracks that you could download as well as X1. There were various people here (myself included) who couldn't get the Dim Pro tracks to play in one of the demos. The way around it was to make a note of the patch name and then load a fresh instance of Dim Pro and then move the midi to it and load the patch.
Just a thought.
 
Grum.
 
P.S. When I went to 64 bit from 32 bit it junked just about every track with a plugin so if you are just having Dim Pro trouble you are lucky.

Yep, that error was what I was talking about. That project uses DimPro 32 bit, so that's all Sonar will look for. It doesn't regard DimPro64 as the same thing.

I've got X1 seeing both, but unfortunately I can't recall how. Sorry.

I just installed the 64 bit version of Sonar X1 Producer on my new computer as well, and several DirectX audio effects I used in multiple tracks of previous songs are no longer available.  So if I install the 32 bit version too, I assume I have to open a song in 32 bit to use these effects?  And this means I cannot use these effects going forward unless I flip the song back and forth between 32 & 64 bit programs?

I learned more about this subject here in four minutes than I have in four years.
 
Thank you