Does the effect from mjolnir apply when you equip it in offhand?

[HNTR]

I've been using it in mainhand always with a shield (the parry timer feels better on that one and for offhand weapons I have to hold the key for attacks which is clunky) on full thor armor for more stun. However I've found out about other weapon builds because I felt like I'm not min maxing enough/being most efficient. I know you don't really need to in this game but that behavior of min maxing everything has seeped in from other games.
I was considering a spear (Gungnir or Gae Bolg) since everyone says they're the best weapons. I really need some more stun and damage to quickly deal with enemies on river raids before my crew dies because they charge ahead like headless chickens often setting themselves on fire or facetanking every attack. I would rather just solo the raids but I fear I'll get desynchronized as it's not intended to do it like this.
I saw on a reddit comment with 5 upvotes that the passive effect of mjolnir applies when in offhand. Others have also recommended Gae Bolg in offhand. Now I'm kinda confused how this should work. How does the offhand system work with a weapon equipped? I only used shields before on that slot.