Absolutely. With DSM you can have a central designtime system, let's say system A. When creating decision services and business rules, you can access backend data of any systems in your system landscape. So there's no need for data replication for designing rules. Decision services then can be deployed to any systems of your system landscape, including system B, where they can be processed, and have access to all DDIC data, ABAP methods and function modules.
For further details please refer to http://scn.sap.com/docs/DOC-29158. Especially the overview presentation and the 3 video tutorials provide for a good introduction to DSM.
