It's not 100% clear to me what setup you're after, but if you're looking to set up a traditional Authoring/Delivery configuration - one of the few paid offerings Umbraco has is called Courier. It's a very reasonably priced (~$135USD,/99EUR) deployment manager that handles syncing content between two sites, i.e., Authoring and a Delivery server.
It's a very smart tool that manages content, configuration, and dependencies. It's neat and also supports a great open-source project!
If you're looking to setup something more like a centralized product database that is used by many sites - amelvin is on good pointer with BASE. They have a nice api where you may also set up your own webservice (beyond their own webservice functaionality!).
If you need this centralized product data to notify the other sites to update their caches - i encourage you to look into the 'distributedCall' functionality.
There's a bit of documentation on distributed calls in this load-balancing tutorial that may help understand the concept a bit better.
...Hope this helps get pointed in the right direction.