I've read parts of this manga and its spin-off. i was just wondering how exactly these two series connect. are these twins the children of that couple of the guy with the heart disease and the daughter of the doctor?
No, this manga it's not an spin-off. Actually, the spin-off is Boku no Hatsukoi o Kimi ni Sasagu (the guy with the heart disease) The protagonist of this manga is in the same dorm as the guy with the heart disease.