Is there a setting that allows two instances of Platinum, or one of Platinum and one of X3 open at the same time? I have "share drivers with other programs" enabled but that doesn't seem to help. I get a message saying "no audio devices available" in the second program.
 

Assuming that they have not changed this, the setting for allowing more than one project open at a time can be found under:
Preferences > File > Advanced
 

Thanks Kev. I looked there on your advice and the option to have only one project open is unchecked (ie will allow 2). However, just to clarify, I actually want to have two instances of Sonar running - not two projects within the same instance. I want to be able to alt-tab between two projects to copy/paste some tracks.

You can copy/paste track content (not tracks) when you have two projects open in a single instance. You can arrange them in columns for easier dragging and dropping. There is no need to have two instances of Sonar open for this and in fact you can't.

Thanks Sanderxpander. If I can't - that is a real shame for me. I'm having trouble transferring my projects from X3 to Platinum because of Bundle file and 'save as' files issues, so was hoping that I could do it this way. 

Ok, what issues are you having with save as?
 
It should be relatively straight forward to do what you want.
 
Are you attempting to open a bundle file or a cwp?
 
If it's the latter, what happens if you start Platinum and open your project from there?
 
This is by far the easiest method so if it's not working for you we need to try and find out why.

Hello Jonesey, I've got a whole bunch of weird stuff going on! 
 
I have a whole album of songs about 2/3 of the way completed, but I had so many issues with X3(E) that I resorted to buying a new computer, upgrading to Platinum, and upgrading Auto tune (which seemed to be related to some issues) just in the hope that things improved. I've tried saving as a new project in Platinum and get things like this ...
https://www.dropbox.com/s/n8lheagq95j6fck/2015-02-10_2.bmp?dl=0
where the pro channel is a visual duplicate of the track!!! ... I have tried saving as a bundle file and then reopening as a new project in a new location but the bundle file is somehow still linked to audio files that were long ago removed from the project. To test out what was happening I incrementally reduced the size of a saved copy of the project and ended up with only one 5 sec piece of audio (trimmed and bounced) which still when bundled, listed as 1.4GB. Go figure??!!! 
 
So, the plan was to try copy/pasting from the old project to a brand new Platinum project, piece by piece. Looks like that is not an option. I'm continuing to try different things as I write ...

X3e is really quite stable. I'd suggest getting it working properly, opening your projects in X3e, and "save as" using per-project folders. Then open those in Platinum. I've had no problem opening files created in X3e but then again, didn't have problems with X3e originally.

To the original question, you should be able to run X3 and Platinum side by side with share drivers enabled. That said, I notice you're running an Octacapture, and I have the same issue on my laptop using a Duo Capture EX, whereas sharing works fine on my desktop with the MOTU interface. So it may be that the Roland drivers just don't play nice with this.
 
As others have suggested, however, you can open two projects in the same instance of SONAR and Ctrl-tab between them instead Alt-tabbing, or even tile the windows and drag content from one to the other.
 
But, ultimately,  I would suggest you use the forum and/or Cakewalk Tech Support to get to the bottom of why you can't open old projects and bundles in Platinum, and get them working normally.
 
The Inspector pane redraw thing has come up a few times. Back in X1 or X2, there was a problem with having the Inspector locked to a bus when the project was saved. You might want to check for that.

There is no need to share drivers, simply set one to MME and use the built in soundcard driver for the purpose of copying tracks/events.