There's an Alden 200 loader that takes 200 feet. Those show up on ebay pretty regularly for $25 or so. With one of those you could (somehow) split the roll into equal-sized halves (pretty easy to tell that you have made them equal, even in the dark) then load one into the loader and put the other back into the bag and can for when the first one runs out.

As to how to get it split without at least a room you can make pitch black, I can't help you, but I bet others will chime in with ideas.