I am planning to work on a collaboration project.  I have X2 Producer and the other has Essential --- Essential opens and saves same file types as Producer?  I think it even looks the same more or less right, but just does not have all the kit which Producer has?
We are also using the same soft synth (well, at least one:  Omnisphere - I presume files/sounds/multis etc used by one person can be saved and open by the other, all within a Sonar project? 

If you are limiting the project to only include stuff the other guy has with Essentials then it should be fine as far as I know. The only problems I could see happening is if you exceeded the Essentials track count. Not sure if there are bit depth/samplerate limitations but you should be keeping those in sync anyway. Cheers.

Yes.... collaboration is possible.
 
I used to send the whole project but the other folks didn't have the synths and the FX that I did and vice versa so I found a better, easier way to collab. When I first started collabing I was sending and receiving bundle files .... it was a huge pain and time consuming. Once the file arrived I had to insert new synths and FX for what was missing.....
 
The easier way to work:
 
Send them the wave or even the MP3 of the project as it is exported.  Be sure to tell them the BPM of the project so everything stays lined up.  That way, you can collab using email or a service like drop box to exchange the files and it's so much faster and easier to upload and download the audio file. You can send the entire project as a single wave (mixed to that point) or as individual track waves.... or as MP3 files. Your choice. Using the waves and MP3's you can work across platforms with people who are using something other than Sonar. I recently worked seamlessly with a guy using Pro Tools in his studio.....
 
The only person who really needs the waves is the one who is doing the final mixing.
 
ADD: be sure to include the ENTIRE track... that means any count in at the start. I always start my projects on measure 3 allowing measures 1 & 2 as count in and for any "pick up" notes.... do not trim the files.....also allow several measures of silence at the ends for reverb trails to fade to silence.

+1 to Herbs solution

Aha ... thanks for the replies.  Guitarhacker, some good ideas ... so basically the plan is to make tracks and render as audio, to which the other user will add new tracks and render as audio etc.    Then one person will do the final mix and create a final WAV for mastering?   This way any DAW could be used and any soft synth combinations ... I guess then if, say, using dropbox, then it could be a bunch of equal length stems in there for each song, that would need to be imported each time ... which sounds a little clunky compared to sending a .cwp file which includes the stems in place ... or did I misunderstand the process?

I deal with people sending me Sonar projects and i have them send me the CWB. If they have any non native effects they want  on their tracks, i asked them to bounce them and i never ran into an issue of Effects missing on tracks.
 
CJ

Really unless the person you are sending it to needs to adjust specific levels a simple stereo wave or mp3 is fine. If you want to use stems then you could just mix down the stems and put them in a new project (using the Copy Audio Folder option so the waves are included or a bundle). That way it's not just a bunch of waves. Also you can have markers, track names and settings ready for them which makes things easier for the recipient.

Cutting a vocal for a local bass player's record.
 
He Emailed a stereo wma file of the music mix.

• Loaded it into X2
• Added an EQ cut at 200Hz (helped clarity for monitoring)
• Added markers for each vocal section (so I could quickly navigate)

Super easy...
I'll Email an mp3 or wma for his approval.
Once approved, I'll upload the finished 24Bit wav files.
 
As has been mentioned, you can collaborate with anyone (any platform).