During the quest Trial of Trinimac, both halves have to be acquired and then assembled at an archaeology station.
The left half is found in the left room locked behind a door that requires the Staff of the Sky Prince and a pedestal that requires Ring of Phynaster to raise the platform.
The right half is found in the right room locked behind a door that requires Staff of the Snow Prince to open. and a pedestal that requires the Warlock's Ring to raise the pedestal.
Remember to take the stone with you when you leave! The temple will become unavailable afterwards and the item will be lost if you forget.
Pedestal with half a touchstone