Hi, I'm new here. Can I ask how can you play/download both the og KoiKatsu and SunShine? Do I just put them all together after downloading both ver?
Koikatsu and Koikatsu sunshine are two differents games. Don't even try to merge them.
Sunshine has a different setting, a different system to increase the girl love meter (if you can call it that), 5 extra personality, a few specific h event animation and a few specific clothing.
If you only want to toy with the studio, stick to KK. If you want to customize your waifu to toy with them, KK is best thanks to more mods avaiable.
If it's "working hard to have the girls fall for you" that you want, KKS probably has the best system.