[From the Patchnotes 2018-10-04 Thursday] "-- when using a style the higher of style spec and base weapon spec is used (for the styled attack only, offhand / unstyled uses base weapon spec) "
Currently the offhand weapon, when it swings, has its defense penetration (as in Weaponskill vs. Block/Parry/Evade) based on the base weapon skill. This is counter intuitive, as there is a specific spec line that should improve wielding two weapons.
My suggestion is to make the offhand Weaponskill calculation based on the DW/CD/LA spec instead, so that having a high skill in the corresponding dual wielding skill improves the offhand as well.