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
There's the Xcode-Multi-Edit-Plugin. It's still not quite like Sublime because the multiple cursors can't be used to edit beyond the selection you made when you started command-d-ing but it's close enough.
The plugin is not updated for Xcode 7.3 but this can be easily fixed by adding
to the list of similar UUIDs in <key>DVTPlugInCompatibilityUUIDs</key> in the plugin's Info.plist. (Found at ~/Library/Application\ Support/Developer/Shared/Xcode/Plug-ins/XcodeMultiEdit.xcplugin/Contents/Info.plist)
Another promising plug-in which currently lacks Xcode 7.3 support is Multiplex.
Apple just released Xcode 10 beta with the following feature:
The Xcode Source Editor now supports multi-cursor editing allowing you
to quickly edit multiple ranges of code at once. You can place
additional cursors with the mouse via ⌃+⇧+Click or with column select
(⌥+Click+Drag), or with the keyboard using ⌃+⇧+Up to column select up
or ⌃+⇧+Down to column select down. (12564506)
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!