The way Nero handles it is very similar to the way I run things as well.
When it came to deciding what 'multiclass' skills Tiergan had, I just went with what made sense for his history. Tiergan was a gladiator for a long time. He was taught how to use multiple types of melee weapons. That said, while he's very skilled at using the blade (his primary weapon for most of his career), and pretty good at hand-to-hand combat (lots of fist-fights in his history) to reflect his two favored modes of combat -- Tiergan's just a novice when it comes to other weapon types. He knows where the dangerous end is supposed to go, but other than that - if he does well in combat with an axe or a lance it's purely because of combat experience and not because he actually knows how to handle the weapon.
To compensate for that, Tiergan is absolute rubbish at magic and archery. He just doesn't have the proficiency for casting spells at all and would be more liable to hurt himself than an enemy while attempting to use a bow. Additionally, ranged attackers are his biggest bane in combat. He HATES fighting mages. Archers are only marginally less frustrating, because he carries a shield, but only marginally so.