Hmmm confusing
C919 is a Chinese project and like the ARJ21 and the enlarged version of the ARJ21 like -700 and -900 I would expect it to continue as a COMAC project which it is
So why for C929 they have joint with Russia? Doesn't make sense this will actually complicate the development of the C929 adding in a partner at this late stage delaying things
C929 and C939 are building on the C919 platform maybe the China Russia programme is some other project for a wide body airliner as it just doesn't add up adding Russian into C929
C929 will be a wide body aircraft with a completely different cross-section, so it cannot possibly be an extension of the C919 platform. It will be a new platform in itself, and it is still in the early stages of development. The Russian have more experience in building wide body airliners and high-bypass turbofans while the Chinese have more capital and market capacity to ensure the economic return of a high-investment project. The political ties between the two are also fairly stable. So it totally make sense for them to work together on this.
Even if the Russian suddenly pulls out for some reason, COMAC should still be able to finish it on its own, perhaps with just a little more time and cost. The risk should be minimal.
Like taxiya said, I also do not think they will try to develop two different wide body airliners at the same time. The joint project and the C929 should be the same thing, maybe there will also be a Russian built version with a different model name. This project will certainly not be a Chinese built Il-96.
Besides the civilian market, you know both China and Russia would want a newer, higher performance and more efficient wide-body platform (as compared to Il-76/96/Y-20) for AWACS and aerial refueling etc. in the long run. This project would also fulfill that role.