You need to maxed out, only two girls BUT for a secret cutscene and other sub-stories. You also need to maxed other four girls.
The girls are:
-Saeko
-Eri
-Iroha
-Ririka
-Sumire
-Miyakoshi
Each of these girls need 30 flowers. After you get all of these six girls, you'll triggered a secret cutscene which is let you to know someone that you might know (from the previous game)
SPOILER ALERT!!!
https://imgur.com/gallery/1sKpkI7 (this image contain spoiler secret cutscene)