“Oh boy. Where’s the other one?”
Greg clearly knew all about Garnet being a fusion and even recognized Ruby and Sapphire, so something must have happened earlier that caused Garnet to split up in Greg’s presence. I want to know what that was.
I think it was rose deciding to sacrifice herself so that she and greg can have a child.
Ruby and sapphire spend a long time getting to know each other before becoming garnet permanently judging by what she told jamie. If rose was like “so yeah, I’m gonna die so I can have the child of this human I haven’t known very long by gem standards” Garnet could get pissed. Ruby and sapphire don’t know him and he’s gonna die in the blink of an eye by gem standards. and now he’s gonna be the cause of the death of their leader? That could split them apart due to their different ways of approaching the situation.