in the back row. I'm using Kage, but when I place him in front, it always works. Tested this on lepant's mansion

The rune doesn't seem to activate in any instance where the next action in sequence targets the secondary target or is the secondary target itself.

For example (and sorry if it sounds confusing), in a Spark Rune setup, my Spark character will attack, then let's say my Double-Beat user against his/her first target, and if the character right after the Double-Beat user has targetted the Double-Beat user's secondary target, this character will attack it instead of the Double-Beat character attacking this target or any other, nulling its effect. This happens quite often for me when I Free Will.

Without Spark Rune in play, if the regular order of attack would be the Double-Beat user and then the secondary target, the secondary target will move before the second attack kicks in.

It's all pretty lame, and I think I'd rather just stick with AoE Unite Attacks and Killer Runes to avoid the hassle DB causes in manually setting the order of attack up each time so that there's no problem having it activate. =/