I owe this to Victor Papanek who asked what difference it would make if toilet rolls were an inch less wide? (it would make a huge eco impact, but very little difference - without going into details! - to efficacy in use)
But then there's retooling, the size of loo roll holders blah blah
So here's what you do instead. You cut loo rolls in half at the factory. They package up in the same way. They sit next to each other on the holder at home. They last twice as long. They have half the eco footprint on every measure (even transport miles per usage). And yes it probably still works just as well.
If anyone reading has a toilet tissue client (pref with good paper sourcing/policies - so not Kimberly Clark who is subject of a protest/boycott calls for using virgin fibre from cleacut ancient forests...) this is your green promotion/potential product extension. Would be a nice one for M&S methinks, or Asda.