I am working with a client who is syncing between SQL Server and MySQL containing the exact same schema and data. We want to centralize that data into one database. Other
on the otherhand I think you've already mentioned what possible problems you may encounter when synchronizing 2 db at the same time aside from this two I think it would be the resources. since there are different RDBMS working for the application they would also have a separate resources for each, like when I update a particular record of a user it still needs to check on which resource does it really exist, but I love to hear more from other people out there this is really an interesting topic to discuss. ;)