This is the distinctive core feature that makes Together the unique product that it is. You'll find it fully functional in all Together products including the free Together Whiteboard.
Simultaneous round-trip engineering means that your UML class are always synchronized to the source code that implements them. Change something in a Class diagram and the relevant source code updates immediately. Change the code and the visual model updates to stay in sync. There's no intermediate repository, no batch code generation, and no risk of losing code.
Simultaneous round-trip engineering also works for Sequence diagrams. When objects are linked to classes, class and operation names stay synchronized.
To try out simultaneous round-trip engineering:
Create or open a project. Make sure both the Diagram pane and the Editor pane are visible.
Place a class into a class diagram. Note that its declaration instantly appears in source.
Rename the class in place or in Properties (Alt+Enter). Use a valid name for target language. Observe that the source code updates instantly in the Editor.
Add an Attribute using the Class's speedmenu (New | Attribute). Accept the default name.
Go to the Editor pane and rename the Attribute in source code. Click on the Diagram pane and watch the visual element update.
Add a member using the Class speedmenu. Again, the source code reflects the change immediately.
Add another member, but this time enter the full declaration in place (use correct syntax for the target language).
Add a class and member(s) in source by coding the full declaration. Click diagram and watch the visual update.