I bred my two pets Arrhythmia (https://beastkeeper.com/pet/1565950) and Far From Home (https://beastkeeper.com/pet/1594825) and used Pink Fish Eggs on them hoping that I'd get lucky with twin devs, but only one of them ended up being a deviant. 

Pet children links:

https://beastkeeper.com/pet/1620996

https://beastkeeper.com/pet/1620992

Shouldn't twins be either both deviant or not deviant at all? I just found that a bit strange. 

They are dizygotic (fraternal) twins :) Like regular siblings, just born at the same time. You may notice that they also have different stats because of different genes. Deviance chance is rolled separately for each twin. One was lucky, the other wasn't.

Ah, thank you for the clarification :)