Our company uses Redmine to track tech support issues, but sometimes these issues are escalated to the outsourced development team, who has their own Redmine.
You can implement a simple sync using Redmine Java API http://code.google.com/p/redmine-java-api
There's an application, which can load data from one Redmine and save to another one - http://taskadapter.com , but it only supports interactive (manual) sync, no "daemon syncs".