What is the piano song playing in Sekaiichi Hatsukoi 2, episode 2 when Takano and Onodera viewed the snow scene?
I think I know which song you're talking about. Looking through the OST for season two helped me. I found out that maybe track 6 is the one you're looking for? It's called, " Shunjun suru Kimochi." 

You can check for yourself here.

Although I didn't think that scene has a piano.

But if my suggestion is wrong, the link I provided has the full OST on the playlist. Also, the person's channel has other Sekaiichi Hatsukoi OSTs, you can try your luck there ^^