I have already had a TextEditor for my language extending with syntax highlighting, hyperlink detector and etc plugins. Also I have had an EditorPart for visual programming in the same language. How can I merge both editors in one Part with two switchable tabs like in WindowBuilder (tabs "Source" and "Design")?
You use an editor part extending MultiPageEditorPart to create an editor with multiple pages.
This editor has a createPages method where you add the pages to the editor by calling the addPage method.
The New Plug-in Project wizard can actually create a skeleton of such an editor as one of its examples - make sure you select 'This plug-in will make contributions to the UI' on the second page of the wizard. The final page will then include 'Multi-page editor' in the list of examples.