I'm the first one up this morning and ...
HURRAY!!!! There is poop in the litter box behind the tv!
I think it is too small to belong to Puss 1, so it looks like this might be working! Maybe she read this thread last night?
Once she is using it regularly, start moving it gradually until you get it into the kitty's bathroom.
Or if you want to speed up that process, with a bit of her poop/pee in the box move her and the box and big kitty into the kitty's room with food, water, bedding etc. placed as far away from the litter box as possible. Be sure to place her in the litter box a couple of times before you close the door. (As bear said the cat will claw under the door, put down something on the floor he can't damage for the duration.)
This should require only one night and your problem should be solved. (Later you can move some of her 'scented' cat litter into big kitty's litter box and you can probably persuade her to use that and get back to one box.)
Fox (been there done that many times) fyre