The M♡S on Honoka's eraser means Masumi♡Suzuki. It would be weird to mix and match family names and given names, so this is the most likely given the GL twist. If we ignore the twist, since it's Honoka's eraser, it's also implausible to be Miku♡Shuuhei or Maeda♡Suzuki.
This oneshot hits differently when re-read. I was about to call him a dense bastard at first, but being dense might've saved him here. But if these two girls were bi, they could also build a true polyamorous triangle that goes further than what Kanojo mo Kanojo did.