Gimme a hand! (+ %weapon damage and offhand)

For a dual-wielding character - this one specifically, I’m trying to figure out whether the Fleshwarped Casque is going to be giving me +50% weapon damage from both my main hand and offhand, or just one of those? If both then this is probably a reasonable intermediate step until I can find or craft better gear (self-found). If it’s only main or off-hand then that’s probably not so great and maybe I should switch to a Cowl of the Venomblade instead (which I also have kicking around in my stash).

The whole “main-hand/off-hand” thing seems to be inconsistent with items I’ve seen, a lot only do main-hand. The stats on this item aren’t clear, nor does it give an example of the damage it’ll do.