[ThatOneHop]

I personally tested this out with both Fenri and Whulfs, since both had on hit effects. Replicating what Kameron says is it seems to only trigger the first weapon with the Verglas unique basic attacks and ignores the first. Fenri being mainhand only had it do the ice damage and not Whulf's light like it should, and vice versa. It might have something to do with how the unique basic attack is treated, perhaps?