i had two girls last year who were best friends/arch enemies at the very same time. It wasn't easy, they were attracted to each other yet argued and cried when they played. Alot of times, you do have to leave them at it alone, other times you need to intervene and "show them how to play", work on their heartstrings "it hurts ______when you hit him, makes him feel bad" "why dont you give a hug instead". Alot of times if there is a fight over something, the aggressor is asked to kindly give the toy to the other child. (against their will of course) then when they give it away, you can give them praise for being "kind to their friend"