It is best to get 2 litter boxes for 2 cats.

Anyway to answer your question, YES, they can share the litter box.

I have 2 big and 1 medium size litter boxes at home and my cats (2 cats) are free to use any of those. But when you first time bring back the new one, it is common if the current cat seems a little unhappy to see the new comer sharing his/her "properties" including toys or even you.

After a while, they can be best buddies and even sleep and lick each other.

PS- About your friend's comments, I can say that it is also depending on how you train your cats to accept the new friends. It takes some time for cats to get along depending on their behavior too.

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