Hi Mr Liu,
The following picture is an overview of CRM middleware:
The middleware transfer data by adapters. The data can be transfer to R/3 because there is R/3 adapter. But I see there is no special adapter to connect to another CRM. It seems only XIF adapter could be used. I have not seen a scenario exactly as you asked.
Also about MCM scenario, you are right. According to the note, in that scenario multiple CRM systems are not synchronized directly but they can be synchronized by R/3 backend system.
Please also refer to :
http://scn.sap.com/thread/3412935
http://scn.sap.com/thread/1642903
These discussions contain how to migrate the data by XIF adapter.
I hope this helps.
Regards,
Ellen