I want to separate two objects along an axis, but without scaling them. I already tried S X in local mode, but it didn't work.
As long as you're in Object or Pose Mode, you can also use the 'Manipulate Center Points' feature. You can activate it with this button:
When it is active, the rotation and scale tools work differently. They only affect the location of objects, not their size or orientation any more. Remember to uncheck this feature before proceeding!
Simple answer but an answer as this function is surely a bit unknown :
Push/pull allows to separate two or more objects, between them or along an axis.
This command can be found in "object/transform" menu or via "space" and type "push/...".
Edit : this is also available in edit mode.