I do this with files every now and then and even between me and an editor cross the country.
Nothing syncs the project files and only one person can be working on the project at a time. SO this is more of a house keeping thing.
As long as all the footage and files being used for the project are with a single main folder, but you can have folders within this main folder. Each person/system has a copy of all the media.
Then I keep track of the current project by name. It starts as
FCP Project_System01_In01, before I send it to the other person the file changes to
FCP Project_System01_Out01, the other system changes the name to FCP Project_System02_In02, then send it back as FCP Project_System02_Out02, then it becomes FCP Project_System01_In03 and so on. The OUT files are label with Red and the IN with green.
You will need to relink each time the project goes to the other system but if you use the same names and folder structure you link the first file and FCP links the rest.
I have done this on HD multicam projects with 8 angles and 3 TB of footage.