I actually have a spare copy of this game that I put into a rar archive which I uploaded here:
Otoboku 2 Futari no Elder
All you have to do is that once you have all part rar files downloaded put inside a new map and extract from part 1 and it will extract part 2 and 3 aswell. And then start up the OTOBOKU2.ISO & You'll get into a DVD folder where you want to run "otbk2.exe" and then you're done. ^-^
[btw.... DO NOT run/start the otbk2updater.exe file]
This folder also includes a readme route information file.
Aswell of some help screenshots of interface of what some buttons means since its all in japanese afterall due to not being translated yet.
(Also, I forgot to mention that in order to open the so called "Menu" (save, load, etc) while playing in-game you just right click anywhere within background in-game to have it pop up.
Update (2024) - Installation Guide: https://docs.google.com/document/d/1pgCkauUjm3XGIfUdPVgFFlKTsK-k6ub0P38-bzF7jkA/edit?usp=sharing
This includes all steps on how to install otoboku 2 with pictures!