Hello, welcome to BinkyBunny!
Two rabbits, especially two boy rabbits cannot just be put together and expected to get along. They can fight very seriously (even to the death) – so you need to separate them until you can do an appropriate “bonding.” Bonding basically just means getting the two bunnies to get along. This can take many months before they are ready to live together.
Are your bunnies neutered? If not, they will probably continue to mark their territory until neutered. Bunnies leave poops to mark their territory – so since you’ve brought a new bunny into the house, they will both start marking territory with poops to claim it as theirs.
What you need to do first is separate the two bunnies, and put them into different rooms. Hopefully this will reduce the amount of territorial marking. Then the second step is to get both of the bunnies fixed. Then you need to give them a couple weeks to recover from their neutering. Then, you can begin the bonding process. Let us know when you are ready to begin bonding, or if you have any questions about neutering!