Answer: The same letter in pairs!
1nsane - 5th letter "n"
Advent Rising - 5th letter "n"
Age of Wonders - 12th letter "e"
Alone in the Dark: The New Nightmare - 12th letter "e"
Arcanum: Of Steamworks and Magick Obs... - 25th letter "n"
Baldur's Gate: The Original Saga - 25th letter "n"
Beyond Divinity - 14th letter "t"
Castles + Castles 2 - 14th letter "t"
Die by the Sword + Expansion - 1st letter "D"
Duke Nukem 3D: Atomic Edition - 1st letter "D"
Doesn't work for Tex Murphy: Overseer and Unreal Tournament 2004 ECE (though every other pair of titles does have a matching letter in the same position). Back to the drawing board!
EDIT: Ignore that, I miscounted - the E is in the same position! Maybe it's an anagram?