So, I'm a level 9 Howler (Barbarian/Chanter) and while I have chosen 4 weapon proficiencies to my companions, I have only been able to choose 2. Is this a downside of being a multi-class character or is it a bug?

 

1 at creation, more as you level (unless one of your classes is 'Devoted').  Multi gets less of everything, (Empowers, Focus, Divine, etc) and Proficiencies.  On the plus side you don't need proficiencies to use anything, it just gives you the toggled special weapon ability.

I got both multi and single classes to max level and they have the same number of empower (5) and proficiency points (7 weapons + unarmed). Their class resource is less for each class (11 pure, 9/9 multi). Also proficiency is used for some items (%30 grazes converted to hits with proficient weapons type descriptions etc).

1 at creation, more as you level (unless one of your classes is 'Devoted').  Multi gets less of everything, (Empowers, Focus, Divine, etc) and Proficiencies.  On the plus side you don't need proficiencies to use anything, it just gives you the toggled special weapon ability.

 

Depends on class. Some of them, like fighter, has talent to improve proficient weapon with more damage.

1 at creation, more as you level (unless one of your classes is 'Devoted').  Multi gets less of everything, (Empowers, Focus, Divine, etc) and Proficiencies.  On the plus side you don't need proficiencies to use anything, it just gives you the toggled special weapon ability.

While this is technically true, there are some passives that a are tied to "while wielding a proficient weapon" such as the fighter's "Weapon Specialization"