I am an X3 user planning to collaborate with a X2 user.  I know Sonar 8.5 users can't open X3 files, but I was wondering if X? series users can open each other's files.

I just opened an X3 file in X2 and in gave me a pop-up warning about not saving it or I would lose X3 features, and it opened missing effects (the VST3 ones).
 
If you are going to collaborate, I would recommend using stem files to do so.

Thanks Mettelus,
 
I'm wondering if basic MIDI features, like the Drum Map Mgr would remain in tact.  BTW, can you explain "stem" files?
 
Thanks,
KC

This is the standard warning anytime SONAR is opening a project created by a newer version.

I misspoke because I originally grabbed the wrong file! The VST3 effects cannot be loaded into X2, but the tracks are indeed all there, so you may be able to collaborate, but I would definitely create a project with X2 in one name and X3 in the other.
 
Stem files are audio mix downs of tracks. In the export audio dialog, after selecting tracks you can choose "Tracks" instead of entire mix, and the files save with "[name you choose] - [track name] - [track #].wav"
 
But now that I grabbed the correct project, you may be able to share the project folder, but not sure of any issues (other than X2 cannot see X3-specific features). 

King Conga It might be better for you to think about upgrading to X3. 

He has X3, his collaborator is on X2. If the O/P has X2 as well he may want to consider running 2 versions.

ampfixer
He has X3, his collaborator is on X2. If the O/P has X2 as well he may want to consider running 2 versions.

+1, if that route is an option, I would choose that one first.

Swapping files back and forth between versions should not be a problem so long as the user of the newer version doesn't rely on any features not present in the older version. It's possible one or both of you would encounter some minor weirdness with the persistence or appearance of certain GUI features (take lane order comes to mind), but I wouldn't expect any major problems.
 
Incidentally, 8.5 can open X-series projects just fine; you lose big features like Prochannel and elements of the Skylight interface like the Multidock and Screensets, but, again, if they're not used in the newer version, it's not a problem.

ampfixer
He has X3, his collaborator is on X2. If the O/P has X2 as well he may want to consider running 2 versions.

Totally misread that. Thank you for pointing that out. Now all we have to do is tell the other guy to upgrade. LOL