How can 2 or more D2000 systems exchange data? Is is possible even when each of those systems is running a different version of D2000?
This is a supported scenario. Two or more D2000 systems running different versions of D2000 can exchange data (bidirectionally) via D2000 Gateway process. In fact, there are 2 parts of a gateway: a gateway server and a gateway client. They exchange information based on configured REMOTE OBJECTS (one remote object can be a simple object or it can transfer a whole structure).
More in-detail description can be found in our documentation https://doc.ipesoft.com/label/D2DOCEN/vzdialene_objekty
D2000 Gateway can be also used in transparent mode (https://doc.ipesoft.com/display/D2DOCEN/Transparent+gateway) to transfer values of objects based on parent/child relations (e.g. values of all I/O tags on a single station, line or for whole communication process).
Youmay find this mode described in my blog https://d2000.ipesoft.com/blog/communication-in-test-environments .