[Working together seems to be the most popular answer here... but just how would he create a scenario where Anise and Arietta would be forced to work together rather than just try to out-do one another?]

I think your second point sounds like a more likely solution, but how would someone go about creating a scenario like this? They're not the kind of people you can just lock in a room. I'm sure either one of them would just break the door down first.

[Why are all his girlfriends so dangerous...]