Renaming objects, variables, layers, layouts, etc, may result in conflicts. I think the trick here is to update all files where the object, variable, etc appear. So if you change an instance variable's name, you have to update all layout files where that object appears.
Having two people working on the same file (for instance, if both make changes to a layout), one will override the other.
So it's important to make updates very frequently. Making .capx backups when you know the other person worked on the same file is also a good idea because then you can just copy stuff if you lose some work