Ok, so hear me out here: maybe the issue with this plan was that they didn't give the two girls anything to do other than see and remember. They couldn't talk to anyone. They weren't trying to answer some great philosophical question. All they could do is sit there and record data and talk to each other until they went insane because they were human to some extent and humans do need new stimuli and having just one other person to talk to for 80 years doesn't count because you eventually become so similar it is like talking to yourself. A better solution would be if the Eternities could talk to earth, in order to help people and offer advice, loving people on earth as much as each other. Then their love could evolve, as they put it, since their relationship with others would constantly change.