Interoperability of engineering tools describes their ability to collaborate with each other across their tool borders, company locations and workflow phases. Therefore it is considered as an important indicator for engineering efficiency - but it is rarely supported by today's heterogeneous industrial software. This paper motivates the need of interoperability, describes the state of the art and its issues, derives requirements and presents a new concept to overcome the mentioned issues. This approach introduces the concepts of Collaboration Objects, Data Ownership and Data Exchange Feedback Loops and utilizes a file based data exchange by means of AutomationML. Finally, the paper describes the developed workflow concept together with required software functionality for a collaboration middleware.