OK, I think I worked it out.
1. back up all the local data at ~/Library/Application Support/Graphisoft/TM Data/AC-<your version>/'the folder of this project under user X'
2. open the file, the system will say it opened from computer b, still open it.
3. now you computer A are officially editing the file under user X
4. quite
5. copy the backuped files to above folder, replace all data.
6. open and done. - this time remember to send local changes to the server.
iMac i7 16G / MBP 13" 2.9 i7 8G / Mac mini's - OS X 10.8 - AC 11/12/13/15 [Australia]