Auras on one character (you or your merc) stack. So the holy fire on flame horizon stacks with the holy fire on a dragon (runeword) shield. This applies to any aura. However, if multiple auras of the same type from different characters (suppose you and your merc both have holy fire) are present, only the higher level aura applies. See the link below and look at comments for an example light zealer build. There are several others in the Character and Strategy Guides section if I recall correctly.
http://annihilus.net/wiki_index.php?...g-Zealer-Guide