I don't know if this true or not, but I always prepare 5 twins before start opening them in one go, I mean if first one is Keena, then I believe the other 3 or 4 there will be at least 1 hart. And the worst for me was the 4th is the Hart.
I don't like the idea of buying 1 open 1, then fail, then wait until sometime(maybe farm crones or whatever before you can get another 1) and buy 1 again to open 1 again, I believe this method is not good, or it just my own opinion, because in my personal experience I never get 5 keenas in a row when opening 5 twins in one go, I used to think it like this, how damn lucky I am if I can get 5 keenas in a row inside 5 twins when I open them in one go. But it's matter of luck in the end.

Perhaps just buy Boss drops it will save you from headache and can also be traded.