I think signature Weapon should work for two melee weapons if you have the two weapon fighting feat.
Rationale:
Weapon focus is modified on Arelith to effect weapon groups instead of individual weapons. I suspect the developers did this to give players a bit of freedom and wiggle room. A longsword and a dagger look cool dual wielded. A handaxe and a whip look totally awesome. These unique looks are effective on Arelith because weapon focus has more flexibility.
There is a fear of missing out aspect with signature Weapon. The feat is an auto include for deep rogues, or very close to it. I don't believe that we should have to choose between making cool looking characters and ability taking essential feats. I do not want to be pigeonholed into taking Grip of Night x2 .
