https://github.com/ToroCraft/NemesisSystem/blob/master/java/net/torocraft/nemesissystem/util/HandleNemesisTraits.java#L38

At level 5, nemeses will earn a second trait.

My vote is to mix it up a bit by a random function like:

chance_for_additional_trait = 1 / (10 * num_of_current_traits)

Also, secondary traits should be used less often, maybe half the time of primary ones or less

Some traits will counter each other, though. Like Double_melee and arrow. I guess their appearance is based on their primary trait, but they get the abilities of other traits

Yes, primary affects there visual weapon.