That is correct. The plugin is hard-coded to assume that the item in the second equipment slot is a shield.
While it is difficult to change this, as several parts of the plugin function by specifically checking the shield for things like animation and block percent, it's not impossible to make a patch for this. Unfortunately, if you have different characters with different equipment loadouts, like some actors have two weapons, and some don't have shield slots, a really basic patch probably wouldn't work.
I'd recommend setting actors who use two weapons to be dual wield type instead of manually giving them two weapon slots, though that is going to make it so that dual wielders can't wear shields at all, so might not exactly fit your needs either.