a new Tachi manga this month in Kirara Forward is called Futakaku Kankei

Airi and Himari are twins
They are always together, though recently Himari has been getting along really well with someone other than Airi...

Air and Himari are twin sisters that had always sported identical hairstyle, clothing and accessories. But not long after entering high school, Himari changed her hairstyle, and started doing a part-time job without consulting Airi. Feeling down, one day Airi sees Himari determined to head for work even when her feet was hurting, explaining that "There is a promise between me and senpai I have to fulfill". Seeing Himari like this, Airi comes up with a proposition...

senpai is a girl