Multiple cursors / highlight next instance of currently highlighted word

It is possible starting from Xcode 10 Beta 3

You can add selections for the next and previous find results using the “Find and Select Next” and “Find and Select Previous” menu commands.

Additionally, you can quickly add selections for the next and previous occurrences of the current selected text using the “Select Next Occurrence” and “Select Previous Occurrence” menu commands.

⌥⌘E to select next occurence of currently selected

⌥⇧⌘E to select previous occurence of currently selected

⌥⌘G to find and select next

⌥⇧⌘G to find and select previous

You can also use the Find menu to find these actions. Of course, feel free to bind it to different hotkeys!

If what you're wanting to do is change the name of a variable, then when your cursor is on the variable press control+command+E and type away. (This is "Edit All in scope")

For editing on a Project scope there is always the Editor > Refactor > Rename menu item