According to the translation from pixiv I was given, it could be either or. (The same thing was said on Nico as well) So, it's up to you to decide who it is.

"Sometimes, when I give Sanae-san a sidetail and take a look, I think she starts to resemble Dai-chan. Also, a long-haired Dai-chan looks like a cosplaying Sanae-san. Proof that they're indistinguishable. Therefore, I'm tagging both."

If it is Dai chan, she's certainly dai enough to be mistaken for Sanae.

StriderTuna said:
If it is Dai chan, she's certainly dai enough to be mistaken for Sanae.

This is definitely Sanae, since she has golden eyes. Dai has blue eyes, otherwise no difference.