Ok so I hate advertisements as much as the next person, but this is one of the few times they've gotten me. I saw an ad for this manga, Monotone Blue, with these two on the cover twining tails and was immediately like "ok yeah, I'll bite". Maybe the recommendation bots are finally learning.

The manga isn't perfect, and I wish it was way way longer than just 1 volume, but of course it was right up my alley. The original came out a couple years back, but the English translation was only released 1-2 months ago, so I guess it still needs time to pick up traction and gain more readers. The creator, Nagabe, also made the more well-known "the girl from the other side".