I would start with the usual litter training advice:
- keep them confined to a small-ish space, like their cage, for a few days so they can get the hang of it
- clean up all pee with a papertowel, place in litterbox, and clean up the spot with vinegar and water
- pick up all poos and put them in the box
- put hay in the box - bunnies like to eat and poo
- make sure your box is big enough - ideally both bunnies should be able to sit in it at the same time; maybe give them two boxes if there's room
Since you just got them, they're probably busy marking their territory and hopefully this will slow down as they get used to it.
The other thing to keep in mind is that it can be very hard to litter train unspayed bunnies and you might have limited success. Do you plan to get them spayed? Spaying will also keep them from fighting - once they reach 5 or 6 months two unspayed females are very likely to start fighting due to hormones even if they seem happily bonded now.
Good luck! I hope this helps.
