They cannot breed with each other if a M and FM, so I would agree it is likely territorial. Get a couple of caves and place them at close to opposite ends of the tank. They may have been fighting over food as they do have somewhat similar diets. The chubby will like more meat and the BN more algae/veggies but they are basically ombivores as the bn is also eating the critters that live in the algae.
The chubby will get c;learly bigger than the BN. So hiding places will be helpfull down the road. Plecos like tight places, so the chubby will be needing a larger cave than the BN in the long run.
edited for typos