For this achievement you have to get both Angel and Devil on the same run. Because the game is horrible, crashes really often, you can´t visit new rooms after you clear the boss and many other problems/glitches it may be hard yo get them both on the same run.
There are 2 ways to get the pets:
1-Buy them from the store (250/350 I've seen both prices).
2-Get them from a chest.
Since the game is really random and never guarantees you can get both, the trick I used is to open the first chest until it gave me one of the pets, if it didn´t, I would quit and restart. That way you only have to search for one more pet rather than 2 (Did the same for the necklaces).
Just be patient and hope you have a lucky run.