There is no mode in Photoshop for that. There is the 3D Tile Drawing, but from what I can tell, people have problems with it. What most people do is just paint one side, make a selection, then do Transform > Flip Horizontally (or Vertically). There are no plug-ins that I can find for Photoshop either. Artists looking for this kind of thing typically use a different program to do their painting and then import into Photoshop for finishing touches or don't use Photoshop at all. Corel Painter has had a symmetry painting feature for years and there's several other programs that can do it as well.