For example, equipping a second pole-arm at level 2 pole-arms skill, gives you a total of +3 (2 + 2*.5) The +1 from level 1 doesn't get doubled, due to rounding I presume. Is this working as intended? There's nothing on it in the wiki, but it certainly seems like it was coded to do exactly this. I tested this with the Swords skill and it's the same.
I believe that is an intended nerf. It used to give you the full passive stat bonuses for both weapons, but that was deemed too powerful. It's been like this for a couple of patches, I think.