When pims aren't kept in a group there tends to be a dominant fish which will be aggressive towards the weaker of the two. They are primarily nocturnal hunters which will hide for most of the day...and come out at night to search for food. Try getting a night light and maybe a bigger tank so you can get more.
They actually do better in larger groups, when you have only two like that one will tend to dominate. Most likely the one that isnt very active is feeling a little bullied by the other.
I have three, and have found that they tend to be less interested in the social ladder when grouped together
i knwo i shouldnt have any nitrite about the pitcus once i turned off the light they came out their hiding place. BUt i have decided to take them back, its best for the fish i like my pitcus but their to dirty for my tank if i had a bigger tank it be alright, but then again id have to fight for that tank at which im doing with my parents.