Is it the compass and the map? You could consider it being under ground, since they are found in chests that are on the ground... heeheehee XD
Close enough. Yes, the riddle is specifically the Dungeon Map. The "brother" mentioned in the riddle is the Compass.
I considered them underground because they're found in dungeons, and dungeons are typically under the ground and such.
Nice job, vatti! ^^
(EDIT)
Okay, since vatti didn't post his riddle, I'll just do another one since I was asked to by some of the guys...
Subject: Enemies (in several games)
We are always apart, never within the same tale. Our armor is alike, our weapons are not. My defense is impenetrable save for my blind spot. His assault is fierce and crushing. Of darkness and iron. Who am I? And who is he?