You can find them in the Tomb of the Unknown King where you have to go in order to get an ID number in order to speak to General Carraway. Anyway, from where you find the ID tag, simply keep going right at every cross-junction until you get to a room with what appears to be a large statue standing on top of a large stone. Approach and it will reveal itself to be Sacred, one of the two brothers. Defeat him and he will get up off the stone and leave. This will make two other rooms accessible.
From here, go to the next cross-junction and keep going right again until you reach a room with a large chain at the back. Go up to it and press X, and Squall should undo the chain, and a section of the back wall will drop down and water will begin flowing into the hole in the middle of the room. Go to the next cross-junction and keep going right until you come to a room with a bunch of cogs on the left side. Approach them and press X, and Squall should remove the wheel that is blocking the mechanism, allowing the water wheel to begin turning.
Head back to the beginning by, you guessed it, repeatedly going right until you reach the exit. Go out and save and then go back inside. This time, keep running forward and you will come to the tomb itself which should now be accessible. Go across the raised bridge across the water and inside the tomb area, and you will find Sacred. He will summon his 'big bro' Minotaur. Defeat the two of them in order to unlock them as a GF. You will also receive a Sacred Card for defeating them, and a Minotaur Card from the ghost of the Unknown King.