Ok so abotu a month ago a bought another UDC to accompany my other one.
the cat is doing fine but on the 3rd week a noticed he/she had a beer belly
i dont feed them out of preportoin.( once a day)
so i dont thik its bloating.
Hes scales arent sticking out, like said in that disease, which i cannot rember the name at the moment.
I have seen the two cats circling each other, adn they both refuge in the same cave.
any answers?

I'm assuming you are talking about upside down cats (Synodontis nigriventris)? If you are, they can't get pregnant, but the female can become gravid. They are egg layers and, from the information I have found, they are seldom bred and when they are, it's usually an accident.

The fish is probably just hogging the food and taking it from the other.