Working on a joint project using different versions of sonar.

X3 will open SPlat projects and vice versa, although I imagine X3 will display a warning (that you can ignore) that the project had been saved in a later version.
 
However, keeping two versions of the same project in sync is still a challenge, even if you're both using identical versions. You'll simplify things by appointing one of you as Keeper of the Project. If it's you, then you'd be sending your collaborator full stereo mixes (which can be MP3s for convenience) and he'll be sending you the individual tracks that he creates. Those tracks will either have their effects burned into the bounce so you don't need to even know what they are, or they'll be left raw so you can do what you like with them. Neither of you will have to worry about having the exact same plugins or project routing. You'll add his submissions to the mix and send him back the new 2-bus mix for approval.
 
Even if your partner is a better mixer, and you decide he should do the final mix, you can defer handing off the project until after all tracks have been added. At that point, track effects should already be baked in, while bus effects will be minimal.
 
In any case, I'd suggest doing a very simple project first in order to establish the process before you get fancy.
 
 

Be aware, Noel posted a message recently that Platinum projects with Aux tracks/Patch points cannot be opened in prior versions.