I can guess what happened. Maybe not exactly WHY it happened, but what the immediate cause is. Try the update.

I'm not sure why it wouldn't be able to use the Party Limit Gauge - I've heard of others using it - so long as the plugins are in the correct order: everything that modifies skill cost should be above this plugin. But then, I also haven't really tested with it much. (There would be a valid reason why it would not be compatible with multiple party members spending Party Limit in the same dualtech - I have warnings against that very thing! - but one party member spending it should work fine. Also, do remember: the dualtech costs the skill cost of the component techs, not the dualtech itself!)

Huh, I guess either ATB got an update or most of my users haven't used ATB, if there are issues like these. (Truth is, my own project uses CTB.)

One way for a dualtech to have a "unique cost", as you put it, is to have a dualtech be its own "component" for at least one of the actors involved. (Exactly one, if you're using Party Limit Gauge, because it's a shared pool.)

(There are other solutions, but they would require edits to the plugin itself, and they would not be quick fixes.)