They appear to be twins, but I suspect there are many groups of similar pairs wandering about, unaware of each other. Many readers immediately assume that Cochlea & Eustachia are human girls, which has yet to be confirmed. Their actions seem to be motivated less by curiosity than boredom and an inclination towards purposeless destruction. Any connate objective remains to be determined.
They never stray apart from each other, out of an unspoken proclivity. Perhaps they keep together because they resemble each other; a mixture of vanity and comfort the foundation of their constant companionship. They seem to consider any creature with dissimilar features as inept or untrustworthy.
I first became aware of them hiding in my walls when I was a youngster. I have no doubt these are the creatures responsible for the frequent disruptions of my belongings and occasionally missing tools. I suspect they also give hypnotic suggestions to my cats......