Nice cats .. My 6 week old nable or his elder brother Minu have no litter training, they do it on their own outside in the ground. However for for nabla, as he is yet learn climbng steps, me or anyone at home has to take her to the ground (i.e. our garden).
However my experience about introducing them might be helpful. For the first week i kept nabla inside my room while minu was free to roam. After that i let nable to roam around the house. Minu when ever came across her avoided her like he has seen a ghost. After 3/4 days minu started to smell nable. Then i put nabla in a basket and put in front of minu, he smelled for a while. I repeated this for 2/3 times before i put nabla in front of minu, i stayed always nearby with preventive measure in case things go south. After a while, ( most probably i kept this things for one week) minu and nabla become familiar. Now Minu basically want to play with nable, however nable avoid minu as he is 1 year old and some time can be forceful. But i think they are not going to fight. I had advantage as minu and nabla are from same mother and one is male another is female.
But if both are from same gender their might be complication as my friend who keep cat around told me. About introducing one cat to another : the basic thing is patience and gradualism. If the first introduction doesn't go well, keep them separated...for one who was aggressive, please don't punish him/her. Remember cats are territorial..you have to give them that. Instead put who was submissive away for a while and try to introduce them once again after a while.