@tig said:

This "one-liner" copy/pasted into the Ruby Console makes all faces in the current selection have the same material front and back... BUT don't use it as an excuse for sloppy modeling

Sketchup.active_model.selection.each{|e|next if e.class!=Sketchup;;Face;e.back_material=e.material if e.material;e.material=e.back_material if not e.material}

If a face has a front-material then it's duplicated on its back, if it doesn't have a front-material the back-material [if any] is duplicated on its front.

It doesn't work on 'nested faces' unless you edit the group/instance and then 'select all', and then run the snippet...

This is very useful! Is there a way to make it run through nested groups and components too? Also I need to look up how to add this, but a single click undo?