In the blog linked on the frontpage
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you can find a wiki, also it is in the "Extras:" on the front page. The "GUIDE WIKI" one:
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| n | name | description | requirements | Advantage Gained |
|---|
| 36 | Long-Lost Twin | You have a long-lost twin | Unlock 2 secret endings | Default starting bonuses. A random girl is chosen to be your secret twin. Seduce and impregnate her for a secret ending. |
| 37 | Friendly Competition | Seems you aren't the only guy sowing seeds this season. | Unlock 2 secret endings | Start 27 MP, 27 SP, & 27 Incubus. Your rival starts with 1 preg. Rival will impregnate one random girl every day. End the season with more impregnations than them for a secret ending. |
...
It's been a while since I played it but there are 17 "Secret Mini Endings" described in the wiki, which I assume you need to have two of to unlock? Presumably you unlocked at least two by accident in non NewGame+ so you could see them before.