TLDR

Morellos does not stack. The spells of a champion with two Morellos will still burn for 5% of the enemy's maximum health per second. The champion still gains double stats that both Morellos provide though.

Full explanation

Morellos is an item that allows a champion's spells to burn the enemy for 5% of the enemy's maximum health per second. Some item passives stack when you put both items on the same champion, while others don't. Morello's passive effect doesn't stack.