Greetings.
That is correct, an actor will only be under the visual effect of one item set at a time. The first set in the list that has enough pieces equipped to be active will be displayed, and any further sets that are activated will have their visual effects ignored.
Given the way the plugin works, I can possibly update it to allow the effects of multiple sets to work together, but only if they're all modifying different sprites (the ring changes the face, while the armor changes the battler, for example) but I have trouble imagining a scenario where that would be useful.