Unfortunately, it's both, as that plugin was never designed with dual wielding in mind.
The issue stems from the way dual wielding works by default in engine. The weapon unleash plugin only expects one attack, (as in the default implementation, dual wielding is still only one attack), and therefore only uses the weapon unleash (or replace attack) from the first weapon.
My plugin works by making the default attack action strike twice, which causes the weapon unleash or replace attacks from the unleash plugin to happen twice, not take one from each weapon.
This is something that could be fixed in an update, but wasn't on my roadmap until just now, and it involves deep diving into another plugin to make it compatible, so I wouldn't expect it anytime soon.