When I first got Harlow, she was kept temporarily in my old guinea pig cage, which was a large pet store cage. 
i lined the bottom of the cage with layers of newspaper and used carefresh paper bedding. it’s pretty expensive, but i always used it for my guinea pigs and then for my bunny. From what I’ve read online, the paper bedding is the most recommended for small critters too.

Potty training harlow was super simple. Once I built her C&C cage and she was on her fleece, i lined her litter box with newspaper and put some of her old bedding in there to use up until i could go get her some litter. She didn’t have any pee accidents the week she was in her cage, and her first night she only had a few poops outside her litter box. The second day there was even less, and the third day there was none. If you keep up on putting their poops in the litter box when they have these accidents and clean up their pee, it should be easier. if really just depends on your rabbits. People have said spaying/neutering helps with litter training, but tbh Harlow has had a LOT of accidents, and her first pee accident. So idk if it’s just because she’s still healing or what. 

Also, you should make sure that your bunnies are both the same sex to prevent accidental pregnancies. 
it’s really hard to tell with young bunnies and it’s very common for people to buy bunnies that weren’t sexed correctly. My Harlow was sold to me as a boy. And they knew “for sure” that she was a boy too. So you she make sure because it can be SUPER difficult to tell when they’re that young. 

I use puppy training pads with hay on top, its cost effective and keeps the smell down pretty well.

Too many soft cecals may mean their current diet is a bit too rich?