What weapons can and can't I transmog?

I've wasted more than a few scrolls on Items I couldn't actually transmog.

Meanwhile, I see people who can transmog one handed weapons on main hand weapons and others who can transmog two mainhand weapons into a main and an off-hand weapon.

Weapon type doesn't even seem to be a requirement, except some specific examples, it seem.
I was unable to transmog a mainhand caster dagger into a one handed mace, but my friend managed to transmog a mainhand caster mace into a one handed axe.

I want to transmog my fist weapons into 2x Hand of Nerub. https://www.wowhead.com/mop-classic/...-hand-of-nerub
Its offhand exclusive, but since it looks the same from both directions, I have a feeling it would look great as a dual wielded option. I'm just not sure it will work, since the rules seem extremely vague when it comes to weapons.