Where is the 2nd 15 attribute point quest in Cantha?

Canni find it anywhere :S

There isnt... somehow the 1st one (and the only one) you did gives you 30 attributes instead of 15.

Press H, it should say: "You have redeemed 200 out of 200 attributes possible" or something like that.

There are two seperate 15 point attribute quests. I believe they are Minister Cho's Estate and Zen Daijun. (not positive as to names as I'm not in game at the monment)

those are missions...

Unwelcome Guests (requirement: mission 2) gotten from Zunraa

Ran Musu Gardens: Lost Treasure (meet Raitahn Nem inside Minister Cho's Estate)

You get the other quest at Seitun Harbor, I believe. It's called "Zunraa: An Unwelcome Guest". You have to go back and talk to the NPC named "Kirin" inside the outpost, and he'll give you the quest.

For the quest, you basically have to walk around the Zen Daijun mission area and make every pool of miasma disappear, and then fight a corrupt kirin at the end. You have to have beaten the mission already, though.

on a hill somewhere inside the harbour outpost is a "shrine of zunraa", click on it and the kirin will spawn, he's usually there already though.

You can do this only once you've done the second mission on the island. After that the Kirin will talk to you. Then you get a ground and talk to the guard by the gate to get in.

Here it is more simply than anyone has said:

Quest number 1: It is got from a guard in Ran Musu Gardens after completing the co-op mission 'Minister Cho's Estate'.
Quest number 2: It is got from the Zunraa/Kirin in Seitung Harbour, after completing the co-op mission 'Zen Daijun'.