Seeing how this is more of a Grey Knight question, I'll pose the question here, rather on the rules forum. Straight to business, according to the wording of the Nemesis Lord WL trait, the wording is this:
"Nemesis Lord: You can add 1 to the Damage characteristic of your Warlord's melee weapon. If you Warlord wields a Relic of Titan that is a melee weapon, chose another Warlord Trait or re-roll this result."
Ok, so directing your attention to the the first part of the sentence (underlined), You can add 1 to the Damage characteristic of your Walord's melee weapon. However, here's the interesting bit - what if he wields two? I.e. a pair of Nemesis Falchions. As he wields two weapons, does that mean the Falchions become D3+2 damage (i,e. 1 falchion would become D3+1, and a pair would become D3 + 2, on the account they are two weapons)? Or would only the one weapon get the +1D? The third possibility is that the falchions do the D3 damage and then you add +2 afterward to the result as both weapons benefit from that trait. This would be incredibly spicy if that combination had a potential of 5 flat damage.