Recommended Level: 16
In order to unlock the first of Temenos and Throné's joint chapters you must first complete the second chapter of their respective stories. For Temenos this means traveling to Canalbrine, a port city on the western continent that's easy to reach if you take a ship to Tato'hana first. Throné has two choices when it comes to a second chapter, one in Oresrush and the other in Winterbloom. Winterbloom is closer to her starting point, but Oresrush is easier to reach, and that chapter has no combat. Your choice.
Once you've unlocked their joint chapter you can begin it by traveling to Flamechurch, Temenos's starting point in the game. If both characters are in your party you'll be prompted to begin the story upon entering the town. If not, you can trigger it - again, with both characters in the party - by visiting the Tavern.
(There's also plenty to see in Flamechurch, if you wish to have a look around. Throné has many opportunities to apply her thieving skills.)
The cleric and the thief, who suit each other surprisingly well, arrive in Flamechurch and begin a hunt for a supposed treasure within the cathedral. First, though, they need information, and the only person who might have any is a Forgetful Old Man, near the stairs leading to the church where Temenos began his story, on the east side of Flamechurch.
Switch to night and have Temenos Coerce the man. Beat him with Temenos's Staff to learn that the man, a Former Carpenter, is nowhere near as forgetful as he lets on. Switch back to day and have Temenos Guide the man. Climb to the cathedral via Flamechurch Pilgrims' Way, enter the cathedral, and proceed to the pontiff's former chamber. It's the second room on the right as you enter the cathedral. The carpenter will push back a bookshelf...
... and reveal a secret passage. Ooo. It will take you to the network of supports above the cathedral. Cross to the west side of the cathedral, checking for a set of stairs that will take you down to a chest containing an Angel's Ring. From here you can climb a ladder to find a room, and inside... is the treasure. Such as it is. This will conclude the chapter, and to see the rest you'll need to complete more of the game.