You can add project references to a parent project using Together. You can view referenced projects using the UML Navigator view. Any referenced projects appear in the referenced projects node of the tree view.
Once a project is referenced, you can add shortcut
elements from the referenced project to the parent project. The shortcut
element appears on the diagram within the parent project with a small lock symbol
in the lower right corner,
and the shortcut icon 
in the lower left corner. It is not possible to create new elements or edit
existing elements of the referenced project in the parent project.
To add a referenced project to an existing project:
Right click the project root in the UML Explorer view, and select Properties from the context menu. The project properties dialog opens.
Select Java Build Path from the properties list on the left.
Select the Projects tab on the right, and choose the appropriate project from the list.
Click OK.
To remove shortcut elements, select the element, and press Delete, or use the Delete from view command from the context menu.
NB: You can also add a referenced project when creating a new Together Java Modeling Project. Update the Java Build Path settings in step three of the wizard.