Hi 張海文 •
Welcome to Microsoft Q&A forum and thanks for using Azure services.
As I understand, you are getting SQL Data Sync error when trying to add sync member.
You have mentioned that only one member is added and error occurs when adding the second.
Could you please try the below workaround mentioned at the official documentation of "Creating Multiple Sync Groups" and see if this helps in your case:
Reference: What is SQL Data Sync for Azure?
Please retry and let us know the results. If it is not helpful, please share with us the configuration details of the Databases used in the SQL Data Sync to further look into the issue.
Hope this will help. Thank you.