Bunnies are peeing outside litter box after bonding

I finally bonded my 2 bunnies and the first few days their litter box habits were perfect! They share a pen and I currently have 1 big litter box that they share. The past few days though they've started peeing and pooping everywhere outside their litterbox. They haven't fought and are getting along great. They also dont mind sharing food, water, and toys so I'm not sure what's changed. Any advice? Should I give them 2 litter boxes or get an even bigger one?