Instructions
Backtrack to the chandelier you used to get to the first key. While standing on the chandelier, face the ledge you jumped from when you were traveling to the chandelier the first time and jump back onto it.
Turn slightly to the right and you will see an alcove similar to the one you used to get to the first key. Stand so you are facing the pillar at the corner, where the wall turns into the alcove and jump toward it to catch a ledge.
Climb along the ledge to get into the alcove and jump from ledge to ledge until you get to the wooden platform. Walk along the wooden platform, then jump across to the next one.
Turn left after jumping to the second wooden platform to find the second key. Jump to grab it, which makes it slide down to open the secret passage.