though he'll do neither very well
He'll actually do very terribly.
You seem to want some security. So I can share some of my experience:
-Having at least one mage in the party is very good, because there are 5 magic schools and only 3 non-magic schools, and magic heals are very important. Without a mage, you will not experience a good portion of the game's mechanics: Manipulating enviromental effects and elemental interactions;
-Creating hybrid classes (that mix 2 or 3 of the offensive attributes: Dex, Str and Int) is very complicated and has a risk of resulting in very weak characters;
-Magic dmg is stronger than physical dmg in the beginning and as enemies tend to have higher elemental resistances at some point, things reverse completely, with phys dmg > mag dmg. Magic in forms of crowd control, buffs and debuffs never stops being very useful;
-Crowd control is one of the most important aspects in fights. Mages are the best for this, Rangers come in the 2nd position;
-Weapon abilities such as 1h, 2h, bow and x-bow become obsolete at around lvl13, because there is another skill that doesn't stack with them and can contribute to more dmg than the others;
-The game becomes progressively easier because of increasing versatility, AP and lower cooldowns on mages. The first map is hard compared to the rest of the game, especially without Pyro and Hydro schools;
-With Crafting and Blacksmithing it's possible to create very powerful equipments, especially STR weapons;
-Lockpicking and Pickpocketing are worthless. Lucky Charm's extra item chance is not worthy;
-You can respec your 2 main characters far in the game, but you'll have to buy all the skill books you want to (re)learn;
-If you end up creating weak characters, you can lower the game difficulty;
-This game has decent replay value, so you can play differently next time.
tl;dr For beginners, it's good to have at least one mage and one physical attacker, and it's complicated to create satisfying hybrid classes. If you end up with a weak party, decrease game difficulty.
