AD2225 observes:

Despite the twin cheapness, the characters of the twins were sufficiently well rounded off that you could tell them apart.

That is quite true, but it only works if the viewer can establish a match between the current behavior on screen and the identity. I had a lot of trouble with that, and had to draw a little diagram with little caricatures of characters. Creators tried to make it easier for me and provided twins with easily identifiable features: Kaoruko had a tail, but Sumireko didn't; Yura combed her hair to the right, but Kira combed it to the left. But despite this, I was unable to remember who was whom.

Unfortunately, the original document was not preserved.