Yes, multiple users can work on the same part at the same time if your using intralink. The function is called integrate and you can attempt to reconcile the differences between 2 models. OK now that you know you can, DON'T. Unless the differences are small it can become an arduous task to integrate the features. Missing parents, children, references, you name it, it can go wrong. Working on 2 assemblies isn't so bad, and as of I2 multiple users can do different sheets of a drawing and then merge them together later.